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

[Ramkumar Ramachandra via llvm-commits]

artagnon 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.

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