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

src/compiler/js-intrinsic-lowering.cc

Index: src/compiler/js-intrinsic-lowering.cc
diff --git a/src/compiler/js-intrinsic-lowering.cc b/src/compiler/js-intrinsic-lowering.cc
index a6f9850d6824e602d9a69a933aaed3a31a71e693..954eaad18e66d301804ceebf393e54f1986aab6e 100644
--- a/src/compiler/js-intrinsic-lowering.cc
+++ b/src/compiler/js-intrinsic-lowering.cc
@@ -298,6 +298,12 @@ Reduction JSIntrinsicLowering::ReduceToObject(Node* 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);
 }