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

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/112673002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18356 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -3640,7 +3640,7 @@ void HAllocate::ClearNextMapWord(int offset) {
     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);
   }