[Dominators] Clean up typedefs in GenericDomTreeConstruction. NFC.

Summary: This patch cleans up GenericDomTreeConstruction by replacing typedefs with usings and replaces `typename GraphT::NodeRef` with `NodePtr` to make the file more readable.

Reviewers: sanjoy, dberlin, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34254

llvm-svn: 305715