[CodeGen] Introduce a VirtRegOrUnit class to hold virtual reg or physical reg unit. NFC (#123768)

LiveIntervals and MachineVerifier were previously using Register to
store this, but reg units are different than physical registers. One
important difference is that 0 is a valid reg unit number, but it is not
a valid phyiscal register.

This patch introduces a new VirtRegOrUnit class that is distinct from
Register. It can be be converted to/from a virtual Register or a
MCRegUnit. I've made all conversions explicit and used assertions to
check the validity.

I also fixed a place in MachineVerifier that was ignoring reg unit 0.

0 files changed, 0 insertions, 0 deletions