Run DCE again after all the other optimizations have run.

On docs this doubles the amount of instructions removed.
diff --git a/compiler/optimizing/constant_folding_test.cc b/compiler/optimizing/constant_folding_test.cc
index 02ad675..e420a62 100644
--- a/compiler/optimizing/constant_folding_test.cc
+++ b/compiler/optimizing/constant_folding_test.cc
@@ -62,7 +62,7 @@
 
   check_after_cf(graph);
 
-  HDeadCodeElimination(graph).Run();
+  HDeadCodeElimination(graph, nullptr).Run();
   SSAChecker ssa_checker_dce(&allocator, graph);
   ssa_checker_dce.Run();
   ASSERT_TRUE(ssa_checker_dce.IsValid());