[llvm] [clang] [RISCV] Add support for experimental Zimop extension (PR #74824)

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

> Please reorganize the patch as @dtcxzyw suggested. :-)
> 
> I didn't notice this extension before, so I may not be asking the right question here: These MOPs can be redefined, then, are we able to schedule them in compiler? Becase we don't know the cost of MOPs if we don't know how MOPs are used. For example, MOPs can be redefined as simple ALU instructions, or, it can be instructions with large cost like DIV/REM. I don't know how to model it now, but I don't think defining Sched resources for MOPs is the right way.

Just checked similar extensions in ARM (like AUT, PAC, BTI, etc.), these extensions are defined in NOP encoding space and their schedule resources will be overrided by `InstRW`.

https://github.com/llvm/llvm-project/pull/74824