Allow for customized graph edge pruning in PostOrderIterator.h

Make it possible to prune individual graph edges from a post-order
traversal by specializing the po_iterator_storage template. Previously,
it was only possible to prune full graph nodes. Edge pruning makes it
possible to remove loop back-edges, for example.

Also replace the existing DFSetTraits customization hook with a
po_iterator_storage method for observing the post-order. DFSetTraits was
only used by LoopIterator.h which now provides a po_iterator_storage
specialization.

Thanks to Sean and Chandler for reviewing.

llvm-svn: 160366