[llvm] [CodeGen] Correctly handle non-standard cases in RemoveLoadsIntoFakeUses (PR #111551)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

I haven't used `LiveRegUnits`, to confirm my guesswork, it's maintaining the set of live-registers as we step backwards through the code and the `available` method tells us whether it's in use?

Looking good, but see inline for a potential unsupported arrangement of debug-info and fake uses.

https://github.com/llvm/llvm-project/pull/111551