Add x86 encoding for SIMD imul

Only i16x8 and i32x4 are encoded in this commit mainly because i8x16 and i64x2 do not have simple encodings in x86. i64x2 is not required by the SIMD spec and there is discussion (https://github.com/WebAssembly/simd/pull/98#issuecomment-530092217) about removing i8x16.

cranelift/wasm/src/code_translator.rs

@@ -1008,6 +1008,10 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
             let a = state.pop1();
             state.push1(builder.ins().ineg(a))
         }
+        Operator::I16x8Mul | Operator::I32x4Mul => {
+            let (a, b) = state.pop2();
+            state.push1(builder.ins().imul(a, b))
+        }
         Operator::I8x16Eq
         | Operator::I8x16Ne
         | Operator::I8x16LtS
@@ -1074,13 +1078,11 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
         | Operator::I16x8AddSaturateU
         | Operator::I16x8SubSaturateS
         | Operator::I16x8SubSaturateU
-        | Operator::I16x8Mul
         | Operator::I32x4AnyTrue
         | Operator::I32x4AllTrue
         | Operator::I32x4Shl
         | Operator::I32x4ShrS
         | Operator::I32x4ShrU
-        | Operator::I32x4Mul
         | Operator::I64x2AnyTrue
         | Operator::I64x2AllTrue
         | Operator::I64x2Shl