[BasicAA] Replace VisitedPhiBBs with a single flag

When looking through phis, BasicAA has to guard against the
possibility that values from two separate cycle iterations are
being compared -- in this case, even though the SSA values may
be the same, they cannot be considered as equal.

This is currently done by keeping a set of VisitedPhiBBs for any
phis we looked through, and then checking whether the relevant
instruction is reachable from one of the phis.

This patch replaces this set with a single flag. If the flag is
set, then we will not assume equality for any instruction part
of a cycle. While this is nominally less accurate, it makes
essentially no difference in practice. Here are the AA stats
for test-suite:

    aa.NumMayAlias  |   3072005 |  3072016
    aa.NumMustAlias |    337858 |   337854
    aa.NumNoAlias   |  13255345 | 13255349

The motivation for the change is to expose the MayBeCrossIteration
flag to AA users, which will allow fixing miscompiles related to
incorrect handling of cross-iteration AA queries.

Differential Revision: https://reviews.llvm.org/D136174