[SCEV] Unwind SCEVUnionPredicate from getPredicatedBackedgeTakenCount [NFC]

For those curious, the whole reason for tracking the predicate set seperately as opposed to just immediately registering the dependencies appears to be allowing the printing code to print a result without changing the PSE state.  It's slightly questionable if this justifies the complexity, but since we can preserve it with local ugliness, I did so.