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

[Philip Reames via llvm-commits]

preames wrote:

I'm mildly of the opinion that we should canonicalize the copy instead.  Not a strongly held opinion or anything.  

Maybe instead of a specific canonicalizeCopyInst, we could introduce a generic optimizeInstruction which is called after mutation?  It could canonicalize the copy, but could also e.g. fold a sh2add x2, x1, x0 to slli x2, x1, 2.  

Alternatively, if we wanted something localized, we could extend the late optimization pass we just added for branches to handle non-branch instructions.

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