[llvm] [MCP] Use MCRegUnit as the key type of CopyTracker::Copies map. NFC. (PR #98277)

[Gábor Spaits via llvm-commits]

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

Even though it has worked with `MCRegister` it is better to have the correct type. The changes look good to me. I don't know if my approval is enough or we should also wait for an approval from @qcolombet .

https://github.com/llvm/llvm-project/pull/98277