[llvm] VPlan/Builder: refine return types (NFC) (PR #108858)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Yes, I'm aware of this. However, I don't know if constant-folding makes sense at the VPlan-level: in practice, we use the existing constant-folder via IRBuilder while executing the plan. For the motivation for this change, see #93998.

Do you know if this would help to simplify any code that uses the result of the builders (e.g. removes unnecessary `cast<>`)? If it helps to simplify it would be great. I don't see VPBuilder including constant-folding any time soon, but if it doesn't help to simplify existing code, it might be better to stick with the more general type for now

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