[WebAssembly] Handle V128 register class in explicit locals pass

Summary:
Also add tests to catch crashes in passes that are not normally run in
tests.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D52959

llvm-svn: 344094