[GraphDiff] Make InverseGraph a property of a GraphDiff.

Summary:
Treating a graph in reverse is a property of the GraphDiff and should instead be a template argument, just like IsPostDom is one for DomTrees.
If it's just an argument to all methods, we could have mismatches between the constructor of the GraphDiff which may reverse the updates when filtering them, and the calls retrieving the filtered delete/insert updates.
Also, since this will be used in IDF, where we're using a DomTree, this creates a cleaner interface for the GraphTraits to use the existing template argument of DomTreeBase.

Separate patch from the one adding GraphDiff, so get a clear diff of what changed.

Reviewers: timshen, kuhar

Subscribers: sanjoy, llvm-commits, jlebar

Differential Revision: https://reviews.llvm.org/D50687

llvm-svn: 339699