Factor the dominator tree calculation details out into DominatorCalculation.h. This

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.

llvm-svn: 42248