[Dominators] Facilitate updates to MachinePostDominatorTree

Summary:
Add getBase accessor so that underlying tree can be
manipulated in a similar manner to MachineDominatorTree.

Reviewers: kuhar, arsenm, hliao, nhaehnle

Reviewed By: kuhar

Subscribers: lkail, mgorny, hiraditya, wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77967