Add element-atomic mem intrinsic canary tests for InstCombine.

Summary:
Add canary tests to verify that InstCombine currently does nothing with the element atomic memory intrinsics for memmove and memset.

Placeholder tests that will fail once element atomic @llvm.mem[move|set] instrinsics have been added to the MemIntrinsic class hierarchy. These will act as a reminder to verify that inst combine handles these intrinsics properly once they have been added to that class hierarchy.

Reviewers: reames

Reviewed By: reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35502

llvm-svn: 308247