change is not useful in and of itself, but it lays the groundwork for combining
the dominator and postdominator implementations.
Also, factor a few methods that are common to DominatorTree and PostDominatorTree
into DominatorTreeBase. Again, this will make merging the two calculation methods
simpler in the future.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42248 91177308-0d34-0410-b5e6-96231b3b80d8