[Attributor] Add flag for undef value to the state of AAPotentialValues

Currently, an undef value is reduced to 0 when it is added to a set of potential values.
This patch introduces a flag for under values. By this, for example, we can merge two states `{undef}`, `{1}` to `{1}` (because we can reduce the undef to 1).

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D85592