Allow constants to be deleted by reinserting them into the graph as needed.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index bd28ba1b74d55da9df2dd5c11cc615a6c4ba4f0b..5eb5c9e1c45a03431492dc84bc63e691a98fcf38 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -338,10 +338,7 @@ class HGraph V8_FINAL : public ZoneObject {
 
   void CollectPhis();
 
-  void set_undefined_constant(HConstant* constant) {
-    undefined_constant_.set(constant);
-  }
-  HConstant* GetConstantUndefined() const { return undefined_constant_.get(); }
+  HConstant* GetConstantUndefined();
   HConstant* GetConstant0();
   HConstant* GetConstant1();
   HConstant* GetConstantMinus1();
@@ -456,6 +453,7 @@ class HGraph V8_FINAL : public ZoneObject {
   bool IsInsideNoSideEffectsScope() { return no_side_effects_scope_count_ > 0; }
 
  private:
+  HConstant* ReinsertConstantIfNecessary(HConstant* constant);
   HConstant* GetConstant(SetOncePointer<HConstant>* pointer,
                          int32_t integer_value);
 
@@ -475,7 +473,7 @@ class HGraph V8_FINAL : public ZoneObject {
   ZoneList<HValue*> values_;
   ZoneList<HPhi*>* phi_list_;
   ZoneList<HInstruction*>* uint32_instructions_;
-  SetOncePointer<HConstant> undefined_constant_;
+  SetOncePointer<HConstant> constant_undefined_;
   SetOncePointer<HConstant> constant_0_;
   SetOncePointer<HConstant> constant_1_;
   SetOncePointer<HConstant> constant_minus1_;