[llvm] [CodeGen] Fix DetectDeadLanes for same-class copies with mismatched widths (PR #226556)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

This is incorrect and trying to handle malformed IR. No register class logic should be done in terms of register sizes 

https://github.com/llvm/llvm-project/pull/226556