[llvm] [MCP][NFC] Refactor eliminateSpillageCopies (PR #208210)

[Gaëtan Bossu via llvm-commits]

https://github.com/gbossu commented:

Sorry for the useless review, I realise `MachineCopyPropagation` is a lot more complex than I remembered. I thought I'd still leave some drive-by comments.
Once I understand what needs to be tracked to allow simplifying those "spillage" copies, I'll probably be able to give more useful feedback 😄 I do salute the refactoring effort though!

https://github.com/llvm/llvm-project/pull/208210