[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340569 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed