[BasicAA] Fix recursive phi MustAlias calculations

[BasicAA] Fix recursive phi MustAlias calculations

With the option -basic-aa-recphi we can detect recursive phis that loop
through constant geps, which allows us to detect more no-alias case for
pointer IV's. If the other phi operand and the other alias value are
MustAlias though, we cannot presume that every element in the loop is
also MustAlias. We need to instead be conservative and return MayAlias.

Differential Revision: https://reviews.llvm.org/D82987