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

[Kai Luo via llvm-commits]

bzEq wrote:

> The motivation is to remove a use of Aliases since it would be nice if we could get rid of that in the long term.

@jayfoad May I know why removing `Aliases` in the long term? After that we can use only `SubReg` to describe register relationships?

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