Clarify side effects of HTransitionElementsKind.

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 6251 matching lines @@
       : original_map_(original_map),
         transitioned_map_(transitioned_map),
         original_map_unique_id_(),
         transitioned_map_unique_id_(),
         from_kind_(original_map->elements_kind()),
         to_kind_(transitioned_map->elements_kind()) {
     SetOperandAt(0, object);
     SetOperandAt(1, context);
     SetFlag(kUseGVN);
     SetGVNFlag(kChangesElementsKind);
-    if (original_map->has_fast_double_elements()) {
+    if (!IsSimpleMapChangeTransition(from_kind_, to_kind_)) {
       SetGVNFlag(kChangesElementsPointer);
       SetGVNFlag(kChangesNewSpacePromotion);
     }
-    if (transitioned_map->has_fast_double_elements()) {
-      SetGVNFlag(kChangesElementsPointer);
-      SetGVNFlag(kChangesNewSpacePromotion);
-    }
     set_representation(Representation::Tagged());
   }
 
   Handle<Map> original_map_;
   Handle<Map> transitioned_map_;
   UniqueValueId original_map_unique_id_;
   UniqueValueId transitioned_map_unique_id_;
   ElementsKind from_kind_;
   ElementsKind to_kind_;
 };
@@ skipping 499 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_