Translate Wasm narrow instructions to CLIF's snarrow and unarrow

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.

cranelift/wasm/src/code_translator.rs

@@ -1559,11 +1559,23 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
             let a = pop1_with_bitcast(state, F32X4, builder);
             state.push1(builder.ins().fcvt_to_sint_sat(I32X4, a))
         }
+        Operator::I8x16NarrowI16x8S => {
+            let (a, b) = pop2_with_bitcast(state, I16X8, builder);
+            state.push1(builder.ins().snarrow(a, b))
+        }
+        Operator::I16x8NarrowI32x4S => {
+            let (a, b) = pop2_with_bitcast(state, I32X4, builder);
+            state.push1(builder.ins().snarrow(a, b))
+        }
+        Operator::I8x16NarrowI16x8U => {
+            let (a, b) = pop2_with_bitcast(state, I16X8, builder);
+            state.push1(builder.ins().unarrow(a, b))
+        }
+        Operator::I16x8NarrowI32x4U => {
+            let (a, b) = pop2_with_bitcast(state, I32X4, builder);
+            state.push1(builder.ins().unarrow(a, b))
+        }
         Operator::I32x4TruncSatF32x4U
-        | Operator::I8x16NarrowI16x8S { .. }
-        | Operator::I8x16NarrowI16x8U { .. }
-        | Operator::I16x8NarrowI32x4S { .. }
-        | Operator::I16x8NarrowI32x4U { .. }
         | Operator::I16x8WidenLowI8x16S { .. }
         | Operator::I16x8WidenHighI8x16S { .. }
         | Operator::I16x8WidenLowI8x16U { .. }