[EarlyCSE] Add tests for negated min/max/abs [NFC]

Summary:
I'm planning to update the hashing logic to recognize their equivalence
in a subsequent change (D62644).

Reviewers: spatel

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62918

llvm-svn: 362657