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 LoopVectorizationPlanner.
Follows https://reviews.llvm.org/D28975 and its tentative breakdown starting with the 1st item "refactor Cost-Model to provide MaxVF and early-exit methods.". The refactoring and Planner proposed in this patch are independent of VPlan though.
No change in output intended.
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