Move parts of the logic used by Reassociate to OverflowTracking
(mergeFlags & applyFlags) and move the definition to Local.h.
For now it just moves the NUW/NSW handling, as this matches the uses in
LICM. I'll look into the FP math handling separately, as it looks like
there's a difference between Reassociate (takes all flags from I, while
LICM takes the intersection of the flags on both instructions).
PR: https://github.com/llvm/llvm-project/pull/140403
c92ff61cee