[Dominators][CodeGen] Clean up MachineDominators

Summary: This is a cleanup patch for MachineDominatorTree. It would be an NFC, except for replacing custom DomTree verification with the generic one.

Reviewers: tstellar, tpr, nhaehnle, arsenm, NutshellySima, grosser, hliao

Reviewed By: arsenm

Subscribers: wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67976

llvm-svn: 373101