[llvm] Reapply "[CodeGen] Introduce `MachineDomTreeUpdater`" (#96846) (PR #96851)

[Nikita Popov via llvm-commits]

nikic wrote:

This change has effectively moved DomTreeUpdater.cpp into DomTreeUpdator.h. Please rework this so that GenericDomTreeUpdater is split into two files, GenericDomTreeUpdater.h providing the interface, and GenericDomTreeUpdaterImpl.h providing the implementation that is only included in the file that explicitly instantiates the template.

https://github.com/llvm/llvm-project/pull/96851