[LV] Consistently use vputils::isUniformAfterVectorization [mostly nfc]

I'd extracted isUniform, and Florian moved isUniformAfterVectorization out of VPlan at basically the same time. Let's go ahead and merge them.

For the VPTransformState::get path, a VPValue without a def (which corresponds to an external IR value outside of VPLan) is explicitly handled above the uniform check.  On the scalarizeInstruction path, I'm less sure why the change isn't visible, but test cases which would seem likely to hit it were already being handled as uniform through some other mechanism.  It would be correct to consider values defined outside of vplan uniform here.