[SCEV][NFC] Factor out common logic into a separate method

SCEV has multiple occurences of code when we need to prove some predicate on
every iteration of a loop and do it with invocations of couple `isLoopEntryGuardedByCond`,
`isLoopBackedgeGuardedByCond`. This patch factors out these two calls into a separate
method. It is a preparation step to extend this logic: it is not the only way how we can prove
such conditions.

Differential Revision: https://reviews.llvm.org/D43373

llvm-svn: 325745