[llvm] [NFC][PowerPC] Use tablegen's MatchRegisterName() (PR #111553)

[Kai Nacke via llvm-commits]

redstar wrote:

> I might be missing something,

`MCRegister RegNo = MatchRegisterName(Name);`

With a brief look at the generated code it seems to me that it matches only lower case names.

https://github.com/llvm/llvm-project/pull/111553