[llvm] [RISCV][MC] Add support for experimental Zimop extension (PR #75182)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> We already have `Zicfiss` support in tree, we should somehow merge the MC implementations since `SSPUSH` and so on use the MOPs. `Zicfiss` instructions that are MOPs can be aliases of `Zimop` instructions.

I'm sorry. I'm wrong as we haven't merged the `Zicfiss` support(https://github.com/llvm/llvm-project/pull/66043) yet.
cc @yetingk 

https://github.com/llvm/llvm-project/pull/75182