[WebAssembly] Add tests for weaker memory consistency orderings

Summary:
Currently all wasm atomic memory access instructions are sequentially
consistent, so even if LLVM IR specifies weaker orderings than that, we
should upgrade them to sequential ordering and treat them in the same
way.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D49194

llvm-svn: 337854