[BasicAA] Don't track visited blocks for phi-phi alias query

We only need the VisitedPhiBBs to disambiguate comparisons of
values from two different loop iterations. If we're comparing
two phis from the same basic block in lock-step, the compared
values will always be on the same iteration.

While this also increases precision, this is mainly intended
to clarify the scope of VisitedPhiBBs.