[Attributor][FIX] Only avoid visiting PHI uses multiple times (PR51249)

AAPointerInfoFloating needs to visit all uses and some multiple times if
we go through PHI nodes. Attributor::checkForAllUses keeps a visited set
so we don't recurs endlessly. We now allow recursion for non-phi uses so
we track all pointer offsets via PHI nodes properly without endless
recursion.

This replaces the first attempt D107579.

Differential Revision: https://reviews.llvm.org/D107798