[Dominators] Clean up GenericDomTree.h. NFC.

Summary:
This patch cleans up GenericDomTree.h by:

- removing unnecessary <NodeT> in DomTreeNodeBase
- removing unnecessary std::move on bools
- changing type of DFSNumIn/DFSNumOut from int to unsigned (since the members were used as unsigned anyway)

The changes don't affect behavior -- everything works as before.

Reviewers: sanjoy, dberlin, chandlerc

Reviewed By: dberlin

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D34229

llvm-svn: 305710