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

[Vikash Gupta via llvm-commits]

vg0204 wrote:

> Rebasing on #140248 should fix one of 2000 failures.

But, doesn't it make incorporates use of lanemask in livein, which is eventually non-exisistent in my patch!

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