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

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

We already have `Zicifss` 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.

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