[llvm] [Thumb] Resolve FIXME: We can trust live-in information from successor blocks now (PR #84111)

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AtariDreams wrote:

> > Also by the thing that changed I was actually referring to the changes that resulted from moving from livephysregs to liveregunits.
> > R6 is reserved anyway, so it is always "live"
> 
> Then you should explain that in your description. I was going off the text of the FIXME.

I think I found why some are missing from the live-in set, and this patch addresses it: https://github.com/llvm/llvm-project/pull/88023

https://github.com/llvm/llvm-project/pull/84111