[SLP] Initial rework for min/max horizontal reduction vectorization, NFC.

Summary: All getReductionCost() functions are renamed to getArithmeticReductionCost() + added basic infrastructure to handle non-binary reduction operations.

Reviewers: spatel, mzolotukhin, Ayal, mkuper, gilr, hfinkel

Subscribers: RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D29402

llvm-svn: 309566