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

[Luke Lau via llvm-commits]

lukel97 wrote:

> > Looks handy. It would be nice to align the naming of this with LLVM IR's `Value::use_empty()`/`Value::user_empty()` though, what do you think?
> 
> I considered it instead of hasNoUsers(), but what would hasUsers() be? I think this inherently does not align with (User|Use), otherwise we'd have choose to keep getNumUsers()?

Typically it's just `!user_empty()`. I'm not sure I'm following why it doesn't align with getNumUsers

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