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

[via llvm-commits]

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

LGTM. Worth having even a weak regression test - the one I suggested above only
pins the debug output, since no computed pressure value changes on AArch64, but it
does catch the invalid cast. Needs a rebase before landing.


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