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

[Lei Huang via llvm-commits]

lei137 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.
> 
> Yes, I think that's the issue.

That behaviour have not changed AFAIK.

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