[WebAssembly] SIMD encoding tests

Modifies existing SIMD tests to also check that SIMD instructions are
lowered to the expected bytes. This CL depends on D50597.

Reviewers: aheejin

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

Differential Revision: https://reviews.llvm.org/D50660

Patch by Thomas Lively (tlively)

llvm-svn: 339712