[VPlan] Use VPlan to check if only the first lane is used.

This removes the remaining dependence on LoopVectorizationCostModel from
buildScalarSteps and is required so it can be moved out of ILV.

It also improves allows us to remove a few unneeded instructions.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D116554