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

[via llvm-commits]

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" 

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