[LoopFusion] Improve collectFusionCandidates() (#182571)
The order of visiting loops in collectFusionCandidates() guarantees that
a new member can only possibly be added to the end of a set.
Also currently `NumFusionCandidates` counts any loop that is added to a
candidate set. Usually large majority of candidate sets have a single
members so they are not really candidates for fusion. Only the second
member of a candidate set and the ones that come after that could be
counted as fusion candidates.
