This showed up as hot spot in a CPU profile collected when running tests.

This makes mypy/test/testcheck.py about 2% faster on my Linux desktop.

Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py): typechecking got 1.07x slower (175.8s -> 188.8s)
(Performance measurements are based on a single noisy sample)