[llvm] [RISC-V] Add P-ext MC Support for More Pair Operations (PR #154088)

[Qihan Cai via llvm-commits]

realqhc wrote:

I'm creating the draft to receive feedback on
1. The naming `RVPPair*` for classes and potentially _p for paired input. 
2. Should I create distinct classes for potential `RVPPairShift_pi` and `RVPShift_pi` or is there a better alternative. (It will become RVP(Pair)Shift(B/H/W)_(r/pi) so there will be a lot of classes created)

Thanks. 

https://github.com/llvm/llvm-project/pull/154088