[llvm] [NFC][VPlan] Mark VPInstruction::VScale as uniformity-preserving (PR #194674)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> > I am happy to switch the patch to that shape if you prefer — it's also closer to how `vputils::isSingleScalar` itself is written. Do you think this is better direction?
> 
> Yeah, that would be better, I think. Hopefully, we'd also get some IR test changes?


Not sure we would get any IR changes, `vscale` will be outside the loop regions 

https://github.com/llvm/llvm-project/pull/194674