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

[Lei Huang via llvm-commits]

lei137 wrote:

> > I think it's worth pointing out that XER was added in [e6fb4e1](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.
That sounds reasonable to me.

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