[WebAssembly] Handle vector IMPLICIT_DEFs.

Summary:
Also reduce the test case for implicit defs and test it with all
register classes.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D53855

llvm-svn: 345794