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

[Jay Foad via llvm-commits]

jayfoad 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!

The reason for doing #140248 is that it makes `test/CodeGen/AMDGPU/atomic_optimizations_local_pointer.ll` behave the same before and after #129847. So #129847 will no longer make this test fail, so it's one less test failure to worry about, and #129847 does not have to include any diffs for this test, which keeps it simpler.

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