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

[Kai Luo via llvm-commits]

bzEq wrote:

> I think it's worth pointing out that XER was added in https://github.com/llvm/llvm-project/commit/e6fb4e1f8ab3c76cbded23fbebdaec969205031f and CARRY set as an alias to it. Initially we only defined the CARRY as that is the only bit within XER that is interesting to the ppc compiler.

If we don't want to remove CARRY, making CARRY a subreg of XER looks viable to me.

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