Implement narrowing instructions for x86 #1957
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Since the Wasm specification contains narrowing instructions (see https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#integer-to-integer-narrowing) that lower to PACKSS*, the x86-specific instruction is not necessary in the CLIF IR.
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.
In order to make it more clear what the incoming types are for this translation (e.g. two `I32X4`s narrow to an `I16X8`), this change explicitly sets the type to which to bitcast (if necessary) the incoming values.
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These changes add two new CLIF instructions
unarrowandsnarrowfor implementing the narrowing instructions defined in the SIMD spec.