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

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index e7ad219964b3946b35e81dbd76011d7e8ff2133f..623f232424d500eed791042d56ca96de442e4978 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -3633,7 +3633,7 @@ 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);
   }