[RISCV][Disassembler] Refactor simple predicate decoders using a template
This replaces the manual boilerplate for DecodeGPRNoX0, DecodeGPRNoX2,
DecodeGPRNoX31, and DecodeGPRPairNoX0 with a universal filtering template
and constexpr predicate functions.
I will need more of these for the RVY patch series, so submitting this NFC
cleanup first.
Pull Request: https://github.com/llvm/llvm-project/pull/198146