[SCEV] Generalize SCEVEqualsPredicate to any compare [NFC]

PredicatedScalarEvolution has a predicate type for representing A == B.  This change generalizes it into something which can represent a A <pred> B.

This generality is currently unused, but is motivated by a couple of recent cases which have come up.  In particular, I'm currently playing around with using this to simplify the runtime checking code in LoopVectorizer. Regardless of the outcome of that prototyping, generalizing the compare node seemed useful.