[WebAssembly] Emit a splat for v128 IMPLICIT_DEF

Summary:
This is a code size savings and is also important to get runnable code
while engines do not support v128.const.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D55910

llvm-svn: 349724