Support for generating vectors for loads with -1 stride

This patch enables vectorization of loops containing backward array
traversal (array stride is -1).

Contributed-by: Chris Jenneisch <chrisj@codeaurora.org>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@204257 91177308-0d34-0410-b5e6-96231b3b80d8