Modify df_iterator to support post-order actions

Summary: This makes a change to the state used to maintain visited information for depth first iterator. We know assume a method "completed(...)" which is called after all children of a node have been visited. In all existing cases, this method does nothing so this patch has no functional changes.  It will however allow a client to distinguish back from cross edges in a DFS tree.

Reviewers: nadav, mehdi_amini, dberlin

Subscribers: MatzeB, mzolotukhin, twoh, freik, llvm-commits

Differential Revision: https://reviews.llvm.org/D25191

llvm-svn: 283391