Rework post dominator information so that we do not have to

Rework post dominator information so that we do not have to
unify all exit nodes of a function to compute post-dominance information.
This does not work with functions that have both unwind and return nodes,
because we cannot unify these blocks.  The new implementation is better
anyway. :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8460 91177308-0d34-0410-b5e6-96231b3b80d8