Reapply "Convert many LivePhysRegs uses to LiveRegUnits" (#84338)

This only converts the instances where all that is needed is to change
the variable type name.

Basically, anything that involves a function that LiveRegUnits does not
directly have was skipped to play it safe.

Reverts
https://github.com/llvm/llvm-project/commit/7a0e222a17058a311b69153d0b6f1b4459414778