[llvm] [VPlan] Support VPWidenCastRecipe in isSingleScalar. (PR #143552)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I had a similar patch at #162340, but @fhahn pointed out the regression. I now see your follow-up, and probably we can merge that with tweaks?

Oh woops I didn't notice your PR sorry about that. But as for the test change, I'm pretty sure VectorCombine will do some similar sort of similar scalarization anyway. So we might as well model it in VPlan too.

https://github.com/llvm/llvm-project/pull/143552