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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> 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)

Sounds good to me: I agree that getNumUsers is misleading, and that we should strip it. Let me know if anything else needs to be done for this patch.

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