[llvm] [RISCV] Add CompressPat for all cases in isCopyInstrImpl (PR #136875)

[Alex Bradbury via llvm-commits]

asb wrote:

I've found that all cases I saw were ultimately the result of MachineCopyPropagation and can be handled cleanly via the suggested optimizeInstruction route. #137973 implements this approach, and I'll close this draft PR in favour of that one.

https://github.com/llvm/llvm-project/pull/136875