[LV] Move selecting vectorization factor logic to LVP (NFC).

Split off from D143938. This moves the planning logic to select the
vectorization factor to LoopVectorizationPlanner as a step towards only
computing costs for individual VFs in LoopVectorizationCostModel and do
planning in LVP.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D150197