Change the Dominators recalculate() function to only rely on GraphTraits

This is a patch by Guoping Long!

As part of utilizing LLVM Dominator computation in Clang, made two changes to LLVM dominators tree implementation:

 - (1) Change the recalculate() template function to only rely on GraphTraits.
 - (2) Add a size() method to GraphTraits template class to query the number of nodes in the graph.

llvm-svn: 145837