Extricate the "reverse" support from the depth-first iterator. This is really

Extricate the "reverse" support from the depth-first iterator.  This is really
a crappy form of post-order traversal which really does not belong here.  While
we are at it, improve documentation and use a vector instead of a stack.

This improves the post dominator analysis pass by ~5%, and probably also helps
other passes as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9084 91177308-0d34-0410-b5e6-96231b3b80d8