[Dominators][CodeGen] Add MachinePostDominatorTree verification
This patch implements Machine PostDominator Tree verification and ensures that the verification doesn't fail the in-tree tests.
MPDT verification can be enabled using verify-machine-dom-info -- the same flag used by Machine Dominator Tree verification.
Flipping the flag revealed that MachineSink falsely claimed to preserve CFG and MDT/MPDT. This patch fixes that.
Reviewers: arsenm, hliao, rampitec, vpykhtin, grosser
Reviewed By: hliao
Subscribers: wdng, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D68235