[IDFCalculator] Use DominatorTreeBase instead of DominatorTree

Summary:
IDFCalculator used a DominatorTree instance for its calculations. Since the PostDominatorTree struct is not a subclass of DominatorTree, it wasn't possible to use PDT in IDFCalculator to compute post-dominance frontiers.

This patch makes IDFCalculator work with a DominatorTreeBase<BasicBlock> instead, which enables PDTs to be utilized.

Patch by Victor Campos (vhscampos@gmail.com)

Reviewers: dberlin

Subscribers: dberlin, llvm-commits

Differential Revision: http://reviews.llvm.org/D13725

llvm-svn: 250320