Revert "Revert "[AAPointerInfo] handle multiple offsets in PHI""

This reverts commit 12696d30.

Reason: This change is dependent on a commit that needs to be rolled
back because it broke the ASan buildbot. See
https://reviews.llvm.org/rGfc21f2d7bae2e0be630470cc7ca9323ed5859892 for
more information.