[llvm] [VPlan] Recognise single-scalar VPInstructions in isUniformAcrossVFsAndUFs (PR #194674)

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

> > > 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

Incorporated the suggestion. Added a VPlanTest and also edited the title of the PR.

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