[SLP] Support for horizontal min/max reduction.

Summary:
SLP vectorizer supports horizontal reductions for Add/FAdd binary operations. Patch adds support for horizontal min/max reductions.
Function getReductionCost() is split to getArithmeticReductionCost() for binary operation reductions and getMinMaxReductionCost() for min/max reductions.
Patch fixes PR26956.

Reviewers: spatel, mkuper, hfinkel, RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27846

llvm-svn: 314101