[LoopUnroll] Fold all exits based on known trip count/multiple

Fold all exits based on known trip count/multiple information from
SCEV. Previously only the latch exit or the single exit were folded.

This doesn't yet eliminate ULO.TripCount and ULO.TripMultiple
entirely: They're still used to a) decide whether runtime unrolling
should be performed and b) for ORE remarks. However, the core
unrolling logic is independent of them now.

Differential Revision: https://reviews.llvm.org/D104203