[llvm] [PowerPC] Remove carry register (PR #142420)

[Ramkumar Ramachandra via llvm-commits]

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

If I understand correctly, CARRY is identical to XER, and I'm unclear on the purpose of the Alias: perhaps for readability?

https://github.com/llvm/llvm-project/pull/142420