Rework dominator and post dominator information so that we do not have to

Rework dominator and 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@8459 91177308-0d34-0410-b5e6-96231b3b80d8