[llvm] [WIP][CodeGen] Modifying MBB's liveins representation as into regUnits (PR #129847)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Overall I quite like the idea of tracking regunit _roots_ instead of regunits themselves. At least it makes this a smaller incremental change.

https://github.com/llvm/llvm-project/pull/129847