[IDF] Teach Iterated Dominance Frontier to use a snapshot CFG based on a GraphDiff.

Summary:
Create the ability to compute IDF using a CFG View.
For this, we'll need a new DT created using a list of Updates (to be refactored later to a GraphDiff), and the GraphTraits based on the same GraphDiff.

Reviewers: kuhar, george.burgess.iv, mzolotukhin

Subscribers: sanjoy, jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D50675

llvm-svn: 340052