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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Also, side question, what is the point of changing the liveins representation? What does it solve/allow?

It's confusing and unclear how to interpret mixed aliasing register entries with the lanemasks. There's also a compile time cost every time we need to translate between the physical register representation and the actual liveness tracking in regunits. Since these are purely used for liveness, they should always be in terms of regunits 

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