Compiler: rework dataflow iterator.

This cl addresses comments from 278630 - rework DatflowIterator
to be a parent class for the various iteration modes.

Change-Id: Ic16c093597e2d754761b4fdfa47f665d8b315542
diff --git a/src/compiler/dex/vreg_analysis.cc b/src/compiler/dex/vreg_analysis.cc
index 4f516fe..08f1f35 100644
--- a/src/compiler/dex/vreg_analysis.cc
+++ b/src/compiler/dex/vreg_analysis.cc
@@ -473,7 +473,7 @@
   }
 
   /* Do type & size inference pass */
-  DataflowIterator iter(this, kPreOrderDFSTraversal, true /* iterative */);
+  PreOrderDfsIterator iter(this, true /* iterative */);
   bool change = false;
   for (BasicBlock* bb = iter.Next(false); bb != NULL; bb = iter.Next(change)) {
     change = InferTypeAndSize(bb);