[llvm] [RISCV] Consolidate some DecoderNamespaces for standard extensions. (PR #128954)

[Sam Elliott via llvm-commits]

https://github.com/lenary commented:

Nice, this looks neater, and is better for explaining when exactly the decoder predicate support is not good enough to avoid issues with conflicting encodings that we have in the backend.

I have one question, and I think there's a typo in the message where you mention Zcmp and Zcmp, one of which I think was supposed to be Zcmt

https://github.com/llvm/llvm-project/pull/128954