[llvm] [AArch64] Refactor predicate register class decode functions (PR #97412)

[Max Beck-Jones via llvm-commits]

DevM-uk wrote:

Hi @MaskRay, I noticed you were the last person to touch llvm/utils/TableGen/DecoderEmitter.cpp before me. Please could you review my changes if you're available or, if not, perhaps suggest others who could?

https://github.com/llvm/llvm-project/pull/97412