[Turbofan] Intrinsic lowering of ToString(<str>) should provide <str>

src/compiler/js-intrinsic-lowering.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/js-intrinsic-lowering.h"
 
 #include <stack>
 
 #include "src/code-factory.h"
 #include "src/compiler/access-builder.h"
@@ skipping 280 matching lines @@
 }
 
 
 Reduction JSIntrinsicLowering::ReduceToObject(Node* node) {
   NodeProperties::ChangeOp(node, javascript()->ToObject());
   return Changed(node);
 }
 
 
 Reduction JSIntrinsicLowering::ReduceToString(Node* node) {
+  // ToString is unnecessary if the input is a string.
+  HeapObjectMatcher m(NodeProperties::GetValueInput(node, 0));
+  if (m.HasValue() && m.Value()->IsString()) {
+    ReplaceWithValue(node, m.node());
+    return Replace(m.node());
+  }
   NodeProperties::ChangeOp(node, javascript()->ToString());
   return Changed(node);
 }
 
 
 Reduction JSIntrinsicLowering::ReduceCall(Node* node) {
   size_t const arity = CallRuntimeParametersOf(node->op()).arity();
   NodeProperties::ChangeOp(
       node,
       javascript()->Call(arity, 0.0f, VectorSlotPair(),
@@ skipping 137 matching lines @@
   return jsgraph_->javascript();
 }
 
 SimplifiedOperatorBuilder* JSIntrinsicLowering::simplified() const {
   return jsgraph()->simplified();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8