Use constant 0 instead of constant null when clearing map words during allocation folding.

src/hydrogen-instructions.cc

 // 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 3615 matching lines @@
   filler_free_space_size_ = store_size;
 }
 
 
 void HAllocate::ClearNextMapWord(int offset) {
   if (clear_next_map_word_) {
     Zone* zone = block()->zone();
     HObjectAccess access = HObjectAccess::ForJSObjectOffset(offset);
     HStoreNamedField* clear_next_map =
         HStoreNamedField::New(zone, context(), this, access,
-            block()->graph()->GetConstantNull());
+            block()->graph()->GetConstant0());
     clear_next_map->ClearAllSideEffects();
     clear_next_map->InsertAfter(this);
   }
 }
 
 
 void HAllocate::PrintDataTo(StringStream* stream) {
   size()->PrintNameTo(stream);
   stream->Add(" (");
   if (IsNewSpaceAllocation()) stream->Add("N");
@@ skipping 775 matching lines @@
       break;
     case kExternalMemory:
       stream->Add("[external-memory]");
       break;
   }
 
   stream->Add("@%d", offset());
 }
 
 } }  // namespace v8::internal