[Attributor] Allow PHI nodes in AAValueConstantRangeFloating

Traversing PHI nodes is natural with the genericValueTraversal but also
a bit tricky. The problem is similar to the ones we have seen in AAAlign
and AADereferenceable, namely that we continue to increase the range in
each iteration. We use a pessimistic approach here to stop the
iterations. Nevertheless, optimistic information can now be propagated
through a PHI node.