[LV] Refactor Cost Model's selectVectorizationFactor(); NFC

Refactoring Cost Model's selectVectorizationFactor() so that it handles only the
selection of the best VF from a pre-computed range of candidate VF's, extracting
early-exit criteria and the computation of a MaxVF upper-bound to other methods,
all driven by a newly introduced LoopVectorizationPlanner.

Differential Revision: https://reviews.llvm.org/D30653

llvm-svn: 297737