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

[Craig Topper via llvm-commits]

topperc wrote:

> @topperc If I am not mistaken Zicfiss extension uses moprr instructions to get ROP functionality. But Zimop does not limit only the use of Zicfiss extension. They can be redefined and assigned other operations.

Right, but won't users be programming to each of the extensions that redefine them rather than some generic interface? We don't have builtins/intrinsics for all of the HINT encodings that can redefined.

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