[SCEV][NFC] Introduce isBasicBlockEntryGuardedByCond

Currently, we have `isLoopEntryGuardedByCond` method in SCEV, which
checks that some fact is true if we enter the loop. In fact, this is just a
particular case of more general concept `isBasicBlockEntryGuardedByCond`
applied to given loop's header. In fact, the logic if this code is largely
independent on the given loop and only cares code above it.

This patch makes this generalization. Now we can query it for any block,
and `isBasicBlockEntryGuardedByCond` is just a particular case.

Differential Revision: https://reviews.llvm.org/D87828
Reviewed By: fhahn