[SLP] Fix PR23510: Try to find best possible vectorizable stores.

Summary:
The analysis of the store sequence goes in straight order - from the
first store to the last. Bu the best opportunity for vectorization will
happen if we're going to use reverse order - from last store to the
first. It may be best because usually users have some initialization
part + further processing and this first initialization may confuse
SLP vectorizer.

Reviewers: RKSimon, hfinkel, mkuper, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39606

llvm-svn: 317821