[MemoryEffects][NFCI] Make the MemoryEffects class reusable

In a follow up we will reuse the logic in MemoryEffectsBase to merge
AAMemoryLocation and AAMemoryBehavior without duplicating all the bit
fiddling code already available in MemoryEffectsBase.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D153305