[llvm] [RISCV] Handle ADD in RISCVInstrInfo::isCopyInstrImpl (PR #81123)

[Alex Bradbury via llvm-commits]

asb wrote:

I'd somehow missed that I'd never merged this after getting a positive review  - apologies (it happens I was working on something else that needs this change). I've tested it on HEAD and added the extract isReg check Craig suggested. I will now merge.

https://github.com/llvm/llvm-project/pull/81123