[llvm] [RISCV] Fix bugs about getting register list of Zcmp push/pop. (PR #66073)

[Alex Bradbury via llvm-commits]

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

I believe this is correct, would appreciate another quick review from someone who's been more active in the other zcmp patches though.

https://github.com/llvm/llvm-project/pull/66073