Add unarrow instruction with x86 implementation

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.

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2070,7 +2070,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
 
         Opcode::AvgRound => unimplemented!(),
         Opcode::Iabs => unimplemented!(),
-        Opcode::Snarrow => unimplemented!(),
+        Opcode::Snarrow | Opcode::Unarrow => unimplemented!(),
         Opcode::TlsValue => unimplemented!(),
     }