…nant-assume, r=WaffleLapkin

[codegen] assume the tag, not the relative discriminant

Address the issue mentioned in <llvm/llvm-project#134024 (comment)> by changing discriminant calculation to `assume` on the originally-loaded `tag`, rather than on `cast(tag)-OFFSET`.

The previous way does make the *purpose* of the assume clearer, IMHO, since you see `assume(x != 4); if p { x } else { 4 }`, but doing it this way instead means that the `add`s optimize away in LLVM21, which is more important.  And this new way is still easily thought of as being like metadata on the load saying specifically which value is impossible.

Demo of the LLVM20 vs LLVM21 difference: <https://llvm.godbolt.org/z/n54x5Mq1T>

r? `@nikic`

Rollup of 19 pull requests

Successful merges:

 - #144400 (`tests/ui/issues/`: The Issues Strike Back [3/N])
 - #144764 ([codegen] assume the tag, not the relative discriminant)
 - #144807 (Streamline config in bootstrap)
 - #144899 (Print CGU reuse statistics in `-Zprint-mono-items`)
 - #144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information)
 - #144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.)
 - #144914 (Add support for `ty::Instance` path shortening in diagnostics)
 - #144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC)
 - #144999 (coverage: Remove all unstable support for MC/DC instrumentation)
 - #145009 (A couple small changes for rust-analyzer next-solver work)
 - #145030 (GVN:  Do not flatten derefs with ProjectionElem::Index. )
 - #145042 (stdarch subtree update)
 - #145047 (move `type_check` out of `compute_regions`)
 - #145051 (Prevent name collisions with internal implementation details)
 - #145053 (Add a lot of NLL `known-bug` tests)
 - #145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols)
 - #145057 (Clean up some resolved test regressions of const trait removals in std)
 - #145068 (Readd myself to review queue)
 - #145070 (Add minimal `armv7a-vex-v5` tier three target)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 19 pull requests

Successful merges:

 - rust-lang#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N])
 - rust-lang#144764 ([codegen] assume the tag, not the relative discriminant)
 - rust-lang#144807 (Streamline config in bootstrap)
 - rust-lang#144899 (Print CGU reuse statistics in `-Zprint-mono-items`)
 - rust-lang#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information)
 - rust-lang#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.)
 - rust-lang#144914 (Add support for `ty::Instance` path shortening in diagnostics)
 - rust-lang#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC)
 - rust-lang#144999 (coverage: Remove all unstable support for MC/DC instrumentation)
 - rust-lang#145009 (A couple small changes for rust-analyzer next-solver work)
 - rust-lang#145030 (GVN:  Do not flatten derefs with ProjectionElem::Index. )
 - rust-lang#145042 (stdarch subtree update)
 - rust-lang#145047 (move `type_check` out of `compute_regions`)
 - rust-lang#145051 (Prevent name collisions with internal implementation details)
 - rust-lang#145053 (Add a lot of NLL `known-bug` tests)
 - rust-lang#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols)
 - rust-lang#145057 (Clean up some resolved test regressions of const trait removals in std)
 - rust-lang#145068 (Readd myself to review queue)
 - rust-lang#145070 (Add minimal `armv7a-vex-v5` tier three target)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 19 pull requests

Successful merges:

 - rust-lang#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N])
 - rust-lang#144764 ([codegen] assume the tag, not the relative discriminant)
 - rust-lang#144807 (Streamline config in bootstrap)
 - rust-lang#144899 (Print CGU reuse statistics in `-Zprint-mono-items`)
 - rust-lang#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information)
 - rust-lang#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.)
 - rust-lang#144914 (Add support for `ty::Instance` path shortening in diagnostics)
 - rust-lang#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC)
 - rust-lang#144999 (coverage: Remove all unstable support for MC/DC instrumentation)
 - rust-lang#145009 (A couple small changes for rust-analyzer next-solver work)
 - rust-lang#145030 (GVN:  Do not flatten derefs with ProjectionElem::Index. )
 - rust-lang#145042 (stdarch subtree update)
 - rust-lang#145047 (move `type_check` out of `compute_regions`)
 - rust-lang#145051 (Prevent name collisions with internal implementation details)
 - rust-lang#145053 (Add a lot of NLL `known-bug` tests)
 - rust-lang#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols)
 - rust-lang#145057 (Clean up some resolved test regressions of const trait removals in std)
 - rust-lang#145068 (Readd myself to review queue)
 - rust-lang#145070 (Add minimal `armv7a-vex-v5` tier three target)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 19 pull requests

Successful merges:

 - rust-lang#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N])
 - rust-lang#144764 ([codegen] assume the tag, not the relative discriminant)
 - rust-lang#144807 (Streamline config in bootstrap)
 - rust-lang#144899 (Print CGU reuse statistics in `-Zprint-mono-items`)
 - rust-lang#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information)
 - rust-lang#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.)
 - rust-lang#144914 (Add support for `ty::Instance` path shortening in diagnostics)
 - rust-lang#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC)
 - rust-lang#144999 (coverage: Remove all unstable support for MC/DC instrumentation)
 - rust-lang#145009 (A couple small changes for rust-analyzer next-solver work)
 - rust-lang#145030 (GVN:  Do not flatten derefs with ProjectionElem::Index. )
 - rust-lang#145042 (stdarch subtree update)
 - rust-lang#145047 (move `type_check` out of `compute_regions`)
 - rust-lang#145051 (Prevent name collisions with internal implementation details)
 - rust-lang#145053 (Add a lot of NLL `known-bug` tests)
 - rust-lang#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols)
 - rust-lang#145057 (Clean up some resolved test regressions of const trait removals in std)
 - rust-lang#145068 (Readd myself to review queue)
 - rust-lang#145070 (Add minimal `armv7a-vex-v5` tier three target)

r? `@ghost`
`@rustbot` modify labels: rollup