[WebAssembly] Replace LOAD_SPLAT with SPLAT_VECTOR

Splats were selected by matching on uses of `build_vector` with
identical elements, but a while back a target independent node for
vector splatting was added.
This removes the WebAssembly specific LOAD_SPLAT intrinsic, and instead
makes SPLAT_VECTOR legal and adds patterns for splat loads.

Differential Revision: https://reviews.llvm.org/D139871