[Loads] Handle simple cases with same base pointer with constant offsets in...

[Loads] Handle simple cases with same base pointer with constant offsets in FindAvailableLoadedValue when AA is null.

Summary:
This will help with devirtualization (store forwarding with vtable pointers in
the presence of other stores into members in the constructor.) During inlining,
we don't have AA.

Reviewers: davidxl

Subscribers: mgorny, Prazek, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71307