[WebAssembly] SIMD comparisons

Summary:
Match the ordering semantics of non-vector comparisons. For
floating point comparisons that do not correspond to instructions, the
tests check that some vector comparison instruction was emitted but do
not care about the full implementation.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51765

llvm-svn: 342064