[SLP vectorizer]: Choose the best consecutive candidate to pair with a store instruction.

[SLP vectorizer]: Choose the best consecutive candidate to pair with a store instruction.

The patch changes the SLPVectorizer::vectorizeStores to choose the immediate
succeeding or preceding candidate for a store instruction when it has multiple
consecutive candidates. In this way it has better chance to find more slp
vectorization opportunities.

Differential Revision: http://reviews.llvm.org/D10445

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243666 91177308-0d34-0410-b5e6-96231b3b80d8