[llvm] [VPlan] Introduce VPValue::user_empty (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.

We can't eliminate getNumUsers unfortunately -- there is code like this:

```cpp
  VPInstruction *I2 =
      new VPInstruction(Instruction::Add, {VPV1, VPV2},
                        VPIRFlags::getDefaultFlags(Instruction::Add));
  EXPECT_EQ(3u, VPV1->getNumUsers());
  VPV1->replaceAllUsesWith(VPV3);
  EXPECT_EQ(3u, VPV3->getNumUsers());
```

For now, let's just go with the modest user_empty improvement.

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