[clang] [llvm] [RISCV] Add MC layer support for XSfmm*. (PR #133031)

[Jiuyang Liu via llvm-commits]

sequencer wrote:

> SiFive's AME proposal locates in OP-V/OP-VE category

Yes, this is a good point.
I do think we should also remove other vendor instruction sets which abused the standard op fields. 

https://github.com/llvm/llvm-project/pull/133031