[VPlan] Sink non-uniform recieps for scalar plans.

In scalar plans, replicate recipes will only generate a single value per
UF, independent of whether they are uniform or not. So don't consider
uniformity for plans with scalar VFs only.

This allows us to handle a few additional cases in VPlan sinking instead
of non-VPlan sinkScalarOperands.

Depends on D133762.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D134218