[GenericDomTree] Change GenericDomTree to use NodeRef in GraphTraits. NFC.

Summary:
Looking at the implementation, GenericDomTree has more specific
requirements on NodeRef, e.g. NodeRefObject->getParent() should compile,
and NodeRef should be a pointer. We can remove the pointer requirement,
but it seems to have little gain, given the limited use cases.

Also changed GraphTraits<Inverse<Inverse<T>> to be more accurate.

Reviewers: dblaikie, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23593

llvm-svn: 278961