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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> For example: in X86, both EAX and RAX have same set of regunits (HAX, AH, AL), as RAX (64bits) has got lower 32 bit subreg i.e. EAX & got nothing to represent upper 32bits. As in #96146, fake high regunits can now be seen. Such instance is quite commonplace. Its same as Qn & Dn case, but it's been handled as for now using artificial HI parts.

The unaddressable parts should get an assigned regunit. e.g. 318c69de52b61d64d5ea113dc2e9f307f7fd4d51 was movement in this direction for AArch64 

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