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

[chenzheng1030]

This is from code review of #104453

See comment: #104453 (comment)

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.

[llvmbot]

@llvm/issue-subscribers-backend-powerpc

Author: Chen Zheng (chenzheng1030)

This is from code review of https://github.com//pull/104453

See comment: #104453 (comment)

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.