[WebAssembly] vNxM.load_splat instructions

Summary:
Adds the new load_splat instructions as specified at
https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#load-and-splat.

DAGISel does not allow matching multiple copies of the same load in a
single pattern, so we use a new node in WebAssemblyISD to wrap loads
that should be splatted.

Depends on D67783.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67784

llvm-svn: 372655