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

[Jay Foad via llvm-commits]

jayfoad wrote:

There are codegen diffs in `test/CodeGen/Thumb2/LowOverheadLoops/`. It looks like this part of `LowOverheadLoop::ValidateLiveOuts` does not cope well with subregs in `liveins()`: https://github.com/llvm/llvm-project/blob/a07cc181cced04850a9e93fc9b57321c8b6ff408/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp#L1017

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