[AAPointerInfo] OffsetInfo: Unassigned is distinct from Unknown

A User like the PHINode may be visited multiple times for the same pointer along
different def-use edges. The uninitialized state of OffsetInfo at the first
visit needs to be distinct from the Unknown value that may be assigned after
processing the PHINode. Without that, a PHINode with all inputs Unknown is never
followed to its uses. This results in incorrect optimization because some
interfering accessess are missed.

Differential Revision: https://reviews.llvm.org/D134704