[WebAssembly] Prioritize splats over v128.consts

Summary:
Splats are fewer bytes than v128.consts, so use them when either could
apply.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51179

llvm-svn: 340569