[llvm] [llvm-exegesis] Begin replacing unsigned with MCRegister. NFC (PR #123109)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM with one comment.

Thanks for taking the time to clean this up. This has been on my todo list for `llvm-exegesis` for a while and I just hadn't gotten around to it.

https://github.com/llvm/llvm-project/pull/123109