[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344094 91177308-0d34-0410-b5e6-96231b3b80d8