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

[Jay Foad via llvm-commits]

jayfoad wrote:

> can you review this, as now only about 2k LIT tests (all across different targets) needs to be updated

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.

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