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

[Vikash Gupta via llvm-commits]

vg0204 wrote:

Also, around 2.2k LIT test need to be updated spanned across multiple archs, so what would be the right way to handle it in terms of PRs, as never dealt with such magnitude of changes before.

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