[VPlan] Update VPInst::onlyFirstLaneUsed to check users. (#80269)

A VPInstruction only has its first lane used if all users use its first
lane only. Use vputils::onlyFirstLaneUsed to continue checking the
recipe's users to handle more cases.

Besides allowing additional introduction of scalar steps when
interleaving in some cases, this also enables using an Add VPInstruction
to model the increment - as a follow up.