This patch combines:

  DenseMap<MachineBasicBlock *, bool> ReachableMap;
  SmallVector<MachineBasicBlock *, 4> ReachableOrdered;

into:

  MapVector<MachineBasicBlock *, bool> ReachableMap;

because we add elements to the two data structures in lockstep, and we
care about preserving the insertion order.

As a side benefit, we get to avoid hash lookups at:

  ReachableMap[MBB] = true;

This relands commit #115111.
Use traditional way to update post dominator tree, i.e. break critical
edge splitting into insert, insert, delete sequence.
When splitting critical edges, the post dominator tree may change its
root node, and `setNewRoot` only works in normal dominator tree...
See

https://github.com/llvm/llvm-project/blob/6c7e5827eda26990e872eb7c3f0d7866ee3c3171/llvm/include/llvm/Support/GenericDomTree.h#L684-L687

Reverts llvm/llvm-project#115111 Causes #119511

Support critical edge splitting in dominator tree updater. Continue the
work in #100856.

Compile time check:
https://llvm-compile-time-tracker.com/compare.php?from=87c35d782795b54911b3e3a91a5b738d4d870e55&to=42b3e5623a9ab4c3648564dc0926b36f3b438a3a&stat=instructions%3Au

Identified with misc-include-cleaner.

This reverts commit c5e5088033fed170068d818c54af6862e449b545.

Causes large compile-time regressions.

Summary:
- Remove wrappers in `MachineDominatorTree`.
- Remove `MachineDominatorTree` update code in
`MachineBasicBlock::SplitCriticalEdge`.
- Use `MachineDomTreeUpdater` in passes which call
`MachineBasicBlock::SplitCriticalEdge` and preserve
`MachineDominatorTreeWrapperPass` or CFG analyses.

Commit abea99f65a97248974c02a5544eaf25fc4240056 introduced related
methods in 2014. Now we have SemiNCA based dominator tree in 2017 and
dominator tree updater, the solution adopted here seems a bit outdated.

`MachinePostDominators` version of #94571.