[llvm] [VPlan] Thread plan to VPBuilder (NFC) (PR #125364)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

We should always have a valid instruction point when we create a recipe I think. Can we get the plan when needed on demand?

Also, it might be worth doing the ConstantFolding in `simplifyRecipe`, so we catch opportunities after RAUW & co?

https://github.com/llvm/llvm-project/pull/125364