[LV] Move getScalarizationOverhead and vector call cost computations to CM. (NFC)

This reduces the number of parameters we need to pass in and they seem a
natural fit in LoopVectorizationCostModel. Also simplifies things for
D59995.

As a follow up refactoring, we could only expose a expose a
shouldUseVectorIntrinsic() helper in LoopVectorizationCostModel, instead
of calling getVectorCallCost/getVectorIntrinsicCost in
InnerLoopVectorizer/VPRecipeBuilder.

Reviewers: Ayal, hsaito, dcaballe, rengolin

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D61638

llvm-svn: 360758