[wasm-simd][scalar-lowering] Fix lowering of returns

If return's input counts did not change, there is no need to change the
node at all.

Bug: chromium:1127620
Change-Id: I16d14a273d44b9fcd0b5a8af74b7a5a2eda569ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2434998
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70183}

src/compiler/simd-scalar-lowering.cc

@@ -1360,8 +1360,7 @@ void SimdScalarLowering::LowerNode(Node* node) {
       break;
     }
     case IrOpcode::kReturn: {
-      // V128 return types are lowered to i32x4, so if the inputs to kReturn are
-      // not Word32, we need to convert them.
+      int old_input_count = node->InputCount();
       int return_arity = static_cast<int>(signature()->return_count());
       for (int i = 0; i < return_arity; i++) {
         if (signature()->GetReturn(i) != MachineRepresentation::kSimd128) {
@@ -1374,24 +1373,17 @@ void SimdScalarLowering::LowerNode(Node* node) {
           continue;
         }
 
-        switch (ReplacementType(input)) {
-          case SimdType::kInt8x16:
-          case SimdType::kInt16x8:
-          case SimdType::kInt64x2:
-          case SimdType::kFloat64x2:
-          case SimdType::kFloat32x4: {
+        // V128 return types are lowered to i32x4.
         Node** reps = GetReplacementsWithType(input, rep_type);
         ReplaceNode(input, reps, NumLanes(rep_type));
-            break;
       }
-          case SimdType::kInt32x4: {
-            // No action needed.
+
+      DefaultLowering(node);
+      // Nothing needs to be done here since inputs did not change.
+      if (old_input_count == node->InputCount()) {
         break;
       }
-        }
-      }
 
-      DefaultLowering(node);
       int new_return_count = GetReturnCountAfterLoweringSimd128(signature());
       if (static_cast<int>(signature()->return_count()) != new_return_count) {
         NodeProperties::ChangeOp(node, common()->Return(new_return_count));