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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I left the code dealing with adding operands to phis as-is (keep IR order for both predecessors and operands during intiial construction, canonicalize when region is introduced), pending adjustment if needed.

All other comments should be addressed. Ping :) 

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