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

[Jay Foad via llvm-commits]

jayfoad wrote:

> should the issue #138552 need to be resolved completely before this works can be taken forward

Yes, I think we need to give leaf registers their own unique regunit, even in the presence of ad hoc aliases.

In the VE case, SX0 has two subregs SW0 and SF0 which alias each other. Currently they all have the same single regunit (number 26) so a regnit-based representation can't even distinguish a superreg from its subregs.

cc @qcolombet 

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