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
6c7e5827ed/llvm/include/llvm/Support/GenericDomTree.h (L684-L687)
This reverts commit 0f8849349ae3d3f2f537ad6ab233a586fb39d375.
Resolve conflict in `MachinePostDominators.h` There is a conflict after
merging #96378, resolved in #96852. Both PRs modified
`MachinePostDominators.h` and triggered build failure.
This commit converts most of `DomTreeUpdater` into
`GenericDomTreeUpdater` class template, so IR and MIR can reuse some
codes.
There are some differences between interfaces of `BasicBlock` and
`MachineBasicBlock`, so subclasses still need to implement some
functions, like `forceFlushDeletedBB`.
