[llvm] [VPlan] Introduce VPValue::has[No]Users (NFC) (PR #203518)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Sounds good overall, this reminded me of https://github.com/llvm/llvm-project/pull/157956, with the goal of removing all uses of `getNumUsers`. 

I think we should go down that direction: adding has(No)Users first, then I can update #157956 to add the remaining helpers to completely remove  explicit `getNumUsers`(which is a bit misleading)

https://github.com/llvm/llvm-project/pull/203518