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

[Lei Huang via llvm-commits]

lei137 wrote:

I think it's worth pointing out that the 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.

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