[PowerPC] Extend folding RLWINM + RLWINM to post-RA.

Summary: We have the patterns to fold 2 RLWINMs before RA, while some RLWINM will be generated after RA, for example rGc4690b00. If the RLWINM generated after RA followed by another RLWINM, we expect to perform the optimization too.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D89855