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