[PowerPC] combine two RLWINM generated from __st[d/w/h/b]cx builtins

This is from code review of https://github.com/llvm/llvm-project/pull/104453

See comment: https://github.com/llvm/llvm-project/pull/104453#discussion_r1721334878

```
RLWINM 3, 3, 4, 28, 31
RLWINM 3, 3, 31, 31, 31
```
===>
```
RLWINM 3, 3, 3, 31, 31
```

Note that the first RLWINM is generated in post RA phase, so this has to be done after RA.