Minor object store optimizations.

- Avoid emitting write barrier when the value is null.
- Do not do a typecheck on an arraystore when storing something that
  was loaded from the same array.

Change-Id: I902492928692e4553b5af0fc99cce3c2186c442a
diff --git a/compiler/optimizing/instruction_simplifier.h b/compiler/optimizing/instruction_simplifier.h
index d74b624..3844d57 100644
--- a/compiler/optimizing/instruction_simplifier.h
+++ b/compiler/optimizing/instruction_simplifier.h
@@ -33,6 +33,7 @@
  private:
   virtual void VisitSuspendCheck(HSuspendCheck* check) OVERRIDE;
   virtual void VisitEqual(HEqual* equal) OVERRIDE;
+  virtual void VisitArraySet(HArraySet* equal) OVERRIDE;
 };
 
 }  // namespace art