[llvm] [VPlan] Refine return types in VPBuilder (NFC) (PR #108858)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, with the formatting fixed. All folding is currently done as VPlan transforms which also naturally handles recipes not constructed via the builder (and opportunities exposed through other folding).

We can always adjust as needed in the future.

https://github.com/llvm/llvm-project/pull/108858