[AAPointerInfo] handle multiple offsets in PHI
Previously reverted in
8b446ea2ba39e406bcf940ea35d6efb4bb9afe95
Reapplying because this commit is NOT DEPENDENT on the reverted commit
fc21f2d7bae2e0be630470cc7ca9323ed5859892, which broke the ASAN buildbot.
See https://reviews.llvm.org/rGfc21f2d7bae2e0be630470cc7ca9323ed5859892 for
more information.
The arguments to a PHI may represent a recurrence by eventually using the output
of the PHI itself. This is now handled by checking for cycles in the control
flow. If a PHI is not in a recurrence, it is now able to report multiple offsets
instead of conservatively reporting unknown.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D138991