[turbofan] Properly allocate constant-folded string.

src/compiler/js-native-context-specialization.cc

Index: src/compiler/js-native-context-specialization.cc
diff --git a/src/compiler/js-native-context-specialization.cc b/src/compiler/js-native-context-specialization.cc
index de22145e1fb8b3e0aef25cf96bd28fa784f54ab1..db38c63048085d70648727e58449017080eb3dc8 100644
--- a/src/compiler/js-native-context-specialization.cc
+++ b/src/compiler/js-native-context-specialization.cc
@@ -695,8 +695,9 @@ Reduction JSNativeContextSpecialization::ReduceKeyedAccess(
       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) {