[AAPointerInfo] fix assertion at the pass-through use of a pointer

[AAPointerInfo] fix assertion at the pass-through use of a pointer

HandlePassthroughUser may sometimes create a new entry for the OffsetInfo of a
user in the OffsetInfoMap. This can invalidate outstanding references into the
map, including the one which needs to be copied into the new entry. This
produces invalid offset info that can trigger assertions.

Fixed this by not using references at this point. The bug was originally
introduced in commit ID 0dc0a441323d41b4860668f38d290579e0de130c.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D140837