Optimise following patterns

```
stp     wzr, wzr, [x2, #0x08]       =>     str     xzr, [x2, #0x08]
```

and

```
stp     wzr, wzr, [x14, #0x20]
str     xzr, [x14, #0x18]           =>     stp     xzr, xzr, [x14, #0x18]
```