[llvm] [CodeGen] Utilizing register units based liveIns tracking in MBB (PR #129847)

[Vikash Gupta via llvm-commits]

vg0204 wrote:

> We do need MIR print/parse changes for the liveins in terms of register units. It may or may not make sense to share code with regunit masks for that.

@arsenm  don't you think it's as a big change (from its impact perspective, even not if implementation) that it can break everything, as unlike just introduction of optional laneBitmask to physeg, as supported now, is an extension which supported backward compatability, but this will need compulsory changes in every test files for all archs, as we are supposedly completely stripping physReg concept, replacing with regunits as block liveins , so what about this?

`Secondly, liveins as the regunit mask in someway (alongwith physReg) & representing them completely as register units apparently seems same but are 2 things as in the latter, the concept of liveins as physReg will vanish irreversibly causing significant downfall at use-site of liveins()`

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