Fix non-determinism in Reassociate caused by address coincidences

Summary:
Between building the pair map and querying it there are a few places that
erase and create Values. It's rare but the address of these newly created
Values is occasionally the same as a just-erased Value that we already
have in the pair map. These coincidences should be accounted for to avoid
non-determinism.

Thanks to Roman Tereshin for the test case.

Reviewers: rtereshin, bogner

Reviewed By: rtereshin

Subscribers: mgrang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59401

llvm-svn: 356803