[AAUnderlyingObjects] Introduce an AA for getting underlying objects of a pointer

[AAUnderlyingObjects] Introduce an AA for getting underlying objects of a pointer

This patch introduces a new AA `AAUnderlyingObjects`. It is basically like a wrapper
AA of the function `AA::getAssumedUnderlyingObjects`, but it can recursively do
query if the underlying object is an indirect access, such as a phi node or a select
instruction.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D141164