[llvm] [MachinePipeliner] Use VirtRegOrUnit instead of Register appropriately (PR #177535)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

Thanks for the comments.

>  In general, any functional change needs a regression test.

Yeah, but for various reasons, it's difficult to write meaningful tests for this part (not just here, but throughout the entire MachinePipeliner as well). This is definitely technical debt, but there isn't much I can do at this point.

https://github.com/llvm/llvm-project/pull/177535