[SCEV] Use isBasicBlockEntryGuardedByCond in isLoopBackedgeGuardedByCond

Previously we tried to using isKnownPredicateAt, but it makes an
extra query to isKnownPredicate, which has negative impact on compile
time. Let's try to use more lightweight isBasicBlockEntryGuardedByCond.

Differential Revision: https://reviews.llvm.org/D92152