Most of the SIMD instructions defined in the CLIF and WASM specifications are unimplemented. As a first step towards filling those in, this issue tracks adding the splat instruction as it is defined in CLIF (https://cranelift.readthedocs.io/en/latest/ir.html#inst-splat) and in WASM (https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#create-vector-with-identical-lanes).