Implement splat instruction for x86

[abrown]

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).

[abrown]

@sunfishcode, if you have some time it may be helpful to comment on where this headed by looking at bnjbvr#1 and bnjbvr#3; since I'm in a holding zone until #799 is merged I have been working off of @bnjbvr's repo. It seems to me that this will shake out some bugs and missing features as you will see in my comments to those PRs and, e.g., #802.