[turbofan] Properly allocate constant-folded string.

src/compiler/js-native-context-specialization.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-native-context-specialization.h"
 
 #include "src/accessors.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
@@ skipping 677 matching lines @@
     // is in element access mode and not MEGAMORPHIC, otherwise there's no
     // guard for the bounds check below.
     if (nexus.ic_state() != MEGAMORPHIC && nexus.GetKeyType() == ELEMENT) {
       // Strings are immutable in JavaScript.
       if (access_mode == AccessMode::kStore) return NoChange();
 
       // Properly deal with constant {index}.
       NumberMatcher mindex(index);
       if (mindex.IsInteger() && mindex.IsInRange(0.0, string->length() - 1)) {
         // Constant-fold the {index} access to {string}.
-        Node* value =
-            jsgraph()->Constant(string->Get(static_cast<int>(mindex.Value())));
+        Node* value = jsgraph()->HeapConstant(
+            factory()->LookupSingleCharacterStringFromCode(
+                string->Get(static_cast<int>(mindex.Value()))));
         ReplaceWithValue(node, value, effect, control);
         return Replace(value);
       } else if (flags() & kDeoptimizationEnabled) {
         // Ensure that {index} is less than {receiver} length.
         Node* length = jsgraph()->Constant(string->length());
         index = effect = graph()->NewNode(simplified()->CheckBounds(), index,
                                           length, effect, control);
 
         // Load the character from the {receiver}.
         value = graph()->NewNode(simplified()->StringCharCodeAt(), receiver,
@@ skipping 907 matching lines @@
   return jsgraph()->javascript();
 }
 
 SimplifiedOperatorBuilder* JSNativeContextSpecialization::simplified() const {
   return jsgraph()->simplified();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8