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

[Quentin Colombet via llvm-commits]

qcolombet 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

I haven't read carefully the whole thread, but I agree with this.

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