[llvm] [RISCV][TII] Add and use new hook to optimize/canonicalize instructions after MachineCopyPropagation (PR #137973)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM - I'm not really a fan of the generic name here, but don't have a better suggestion, and don't want this blocked indefinitely.

Once this lands, I will be posting a follow up patch which reorganizes MCP such that after this simplification, we can further propagate the copy propagation.  I feel the follow on with respect to that is key to motivating this direction.  

https://github.com/llvm/llvm-project/pull/137973