SLPVectorizer: Don't vectorize volatile memory operations

radar://15231682

Reapply r192799,
  http://lab.llvm.org:8011/builders/lldb-x86_64-debian-clang/builds/8226
showed that the bot is still broken even with this out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192820 91177308-0d34-0410-b5e6-96231b3b80d8