Adds a shared `unarrow` instruction in order to lower the Wasm SIMD specification's unsigned narrowing (see https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#integer-to-integer-narrowing). Additionally, this commit implements the instruction for x86 using PACKUSWB and PACKUSDW for the applicable encodings.
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.