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

[Jay Foad via llvm-commits]

jayfoad wrote:

> The PPC XER is not equivalent to carry. The XER also contains overflow (OV) and summary overflow (SO) bits. amongst other things. OV is used by addex and some add, subtract, and multiply instructions with the "O" suffix.

Yes, I understand that in the architecture, "carry" is just one bit in the "xer" register. But looking at the definition of `CARRY` in `PPCRegisterInfo.td`, it is identical to the definition of `XER`. Nothing about the definition or how it is used in the compiler says that it only represents one bit in the full register. So I think it is OK to replace all uses of `CARRY` with `XER`.

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