[llvm] [PowerPC] make LR/LR8 CTR/CTR8 aliased (PR #76926)

[Kai Luo via llvm-commits]

https://github.com/bzEq approved this pull request.

The change looks reasonable to me. `LR` is not a subreg of `LR8`, however they do overlap. Same reason to `CTR` and `CTR8`.

https://github.com/llvm/llvm-project/pull/76926