[llvm] [VPlan] Refactor VPlan creation, add transform introducing region (NFC). (PR #128419)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Moved the new transform to `VPlanConstruction.cpp`, to serve as new location for all things initial VPlan construction related (soon also building the initial VPlan).

https://github.com/llvm/llvm-project/pull/128419