[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Responding to some comments inline regarding phi/predecessor ordering to start with, to clarify. Would be good to decide first where we want the canonicalization of phi/predecessors to happen.

Doing it separately from initial VPlan construction has the benefit of reducing the number of special cases needed during initial construction and shifts the complexity of detecting the preheader/latch to a VPlan transform, currently integrated with introducing regions, but could also be a separate canonicalization.

https://github.com/llvm/llvm-project/pull/129402