Relax TransitionElementsKind DependsOn/Changes dependencies.

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 4173 matching lines @@
 
 class HTransitionElementsKind: public HTemplateInstruction<1> {
  public:
   HTransitionElementsKind(HValue* object,
                           Handle<Map> original_map,
                           Handle<Map> transitioned_map)
       : original_map_(original_map),
         transitioned_map_(transitioned_map) {
     SetOperandAt(0, object);
     SetFlag(kUseGVN);
+    // DependsOn flags for TransitionElementsKinds only effect instruction
+    // hoisting, which could change their location relative to instructions that
+    // have side-effects. GVN duplicate value elimination handles
+    // TransitionElementsKinds specially, ensuring the no-op transitions are
+    // always eliminated.
     SetGVNFlag(kDependsOnMaps);
     SetGVNFlag(kChangesElementsKind);
     if (original_map->has_fast_double_elements()) {
       SetGVNFlag(kChangesElementsPointer);
-      SetGVNFlag(kDependsOnElementsPointer);
       SetGVNFlag(kDependsOnDoubleArrayElements);
     } else if (transitioned_map->has_fast_double_elements()) {
       SetGVNFlag(kChangesElementsPointer);
-      SetGVNFlag(kDependsOnElementsPointer);
       SetGVNFlag(kDependsOnArrayElements);
     }
     set_representation(Representation::Tagged());
   }
 
   virtual Representation RequiredInputRepresentation(int index) {
     return Representation::Tagged();
   }
 
   HValue* object() { return OperandAt(0); }
@@ skipping 412 matching lines @@
 
   DECLARE_CONCRETE_INSTRUCTION(In)
 };
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_