[VPlan] Do not create VPWidenCall recipes for scalar vector factors.

'Widen' recipe are only used when actual vector values are generated.
Fix tryToWidenCall to do not create VPWidenCallRecipes for scalar vector
factors.

This was exposed by D123720, because the widened recipes are considered
vector users.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D124718