[LoopVectorizer] NFC: Propagate ElementCount to more interfaces.

Interfaces changed to take `ElementCount` as parameters:
* LoopVectorizationPlanner::buildVPlans
* LoopVectorizationPlanner::buildVPlansWithVPRecipes
* LoopVectorizationCostModel::selectVectorizationFactor

This patch is NFC for fixed-width vectors.

Reviewed By: dmgreen, ctetreau

Differential Revision: https://reviews.llvm.org/D90879