[SCEV] Extend isImpliedCondOperandsViaRanges to independent predicates (#71110)

As far as I can tell, there's nothing in this code which actually
assumes the two predicates in (FoundLHS FoundPred FoundRHS) => (LHS Pred
RHS) are the same.

Noticed while investigating something else, this is purely an
oppurtunistic optimization while I'm looking at the code. Unfortunately,
this doesn't solve my original problem. :)