[llvm] [PowerPC] Peephole address calculation in TOC memops (PR #76488)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 commented:

I suggest we split this patch into two:
1: refactor for the `PeepholePPC64()` which should be NFC
2: add the peephole support for `ADDItoc` and `ADDItoc8`

Although Github is not good for stack review, splitting this patch into two can make this change easier to review.

Would you please help to do that? @ecnelises Thanks.

https://github.com/llvm/llvm-project/pull/76488