Ensure the graph is correctly typed.

We used to be forgiving because of HIntConstant(0) also being
used for null. We now create a special HNullConstant for such uses.

Also, we need to run the dead phi elimination twice during ssa
building to ensure the correctness.

Change-Id: If479efa3680d3358800aebb1cca692fa2d94f6e5
diff --git a/compiler/optimizing/ssa_phi_elimination.h b/compiler/optimizing/ssa_phi_elimination.h
index b789971..88a5279 100644
--- a/compiler/optimizing/ssa_phi_elimination.h
+++ b/compiler/optimizing/ssa_phi_elimination.h
@@ -34,6 +34,9 @@
 
   void Run() OVERRIDE;
 
+  void MarkDeadPhis();
+  void EliminateDeadPhis();
+
  private:
   GrowableArray<HPhi*> worklist_;