Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7486 matching lines @@
   HForInCacheArray* index_cache_;
 };
 
 
 class HLoadFieldByIndex V8_FINAL : public HTemplateInstruction<2> {
  public:
   HLoadFieldByIndex(HValue* object,
                     HValue* index) {
     SetOperandAt(0, object);
     SetOperandAt(1, index);
+    SetChangesFlag(kNewSpacePromotion);
     set_representation(Representation::Tagged());
   }
 
   virtual Representation RequiredInputRepresentation(int index) V8_OVERRIDE {
     return Representation::Tagged();
   }
 
   HValue* object() { return OperandAt(0); }
   HValue* index() { return OperandAt(1); }
 
   virtual void PrintDataTo(StringStream* stream) V8_OVERRIDE;
 
   virtual HType CalculateInferredType() V8_OVERRIDE {
     return HType::Tagged();
   }
 
   DECLARE_CONCRETE_INSTRUCTION(LoadFieldByIndex);
 
  private:
   virtual bool IsDeletable() const V8_OVERRIDE { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_