Add PostDominatorTree::getDescendants.

This patch adds the counter-part to DominatorTree::getDescendants.
It also fixes a couple of comments I noticed out of date in the
DominatorTree class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195778 91177308-0d34-0410-b5e6-96231b3b80d8