Fix instruction removal for 'as' test; the bug caused us to stop optimizing after an 'as' test.

runtime/vm/flow_graph_optimizer.cc

Index: runtime/vm/flow_graph_optimizer.cc
===================================================================
--- runtime/vm/flow_graph_optimizer.cc	(revision 31394)
+++ runtime/vm/flow_graph_optimizer.cc	(working copy)
@@ -3424,7 +3424,8 @@
       }
       // Remove call, replace it with 'left'.
       call->ReplaceUsesWith(left);
-      call->RemoveFromGraph();
+      ASSERT(current_iterator()->Current() == call);
+      current_iterator()->RemoveCurrentFromGraph();
       return;
     }
   }