[LoopUnroll] Push runtime unrolling decision up into tryToUnrollLoop()

Currently, UnrollLoop() is passed an AllowRuntime flag and decides
itself whether runtime unrolling should be used or not. This patch
pushes the decision into the caller and allows us to eliminate the
ULO.TripCount and ULO.TripMultiple parameters.

Differential Revision: https://reviews.llvm.org/D104487