[BasicAA] Include MayBeCrossIteration in cache key

Rather than switching to a new AAQI instance with empty cache when
MayBeCrossIteration is toggled, include the value in the cache key.

The implementation redundantly include the information in both sides
of the pair, but that seems simpler than trying to store it only on
one side.

Differential Revision: https://reviews.llvm.org/D136175