[llvm] [CodeGen][ARM] Test validity of SubRC + SubReg in shouldRewriteCopySrc (PR #159600)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I ran into this issue again, and I don't understand what ARM is doing. getExtractSubregInputs is returning a register + subregister pair, where the class of the register doesn't support that subregister index. I suppose that sort of makes sense, but it requires re-constraining the register if it actually folds into a use (but that's not done currently) 

https://github.com/llvm/llvm-project/pull/159600