IndVarSimplify: thread CmpPredicate to SCEV (NFC) (#125240)
Relevant parts of ScalarEvolution's API accept a CmpPredicate instead of
a CmpInst::Predicate after 60dc450 (SCEV: migrate to CmpPredicate
(NFC)). After auditing the callers of these APIs, it was found that
IndVarSimplify was dropping samesign information. Fix this.