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

[Vikash Gupta via llvm-commits]

vg0204 wrote:

> 2000 is still a lot! We need to understand which ones only fail because the way liveins are printed in MIR has changed (these ones can simply be updated) and which ones actually have different codegen, or fail in some other way.

There are observed only 2 types of failure:

1. Codegen diff (Exit Code : 1) -> either due tu way liveins got printed or some real codegen changes as suggested by you.
2. MachineVerifier failure (Exit Code : 2) -> due to use of register whose regUnits are completely killed in some other form (its subreg or alias). There are only such 2 tests, that can be easily modified to solve it keeping it semantically same.

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