Reapply: Add slp vectorization to LTO passes. The bug it exposed has been fixed by r207983. <radar://16641956>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208013 91177308-0d34-0410-b5e6-96231b3b80d8
Walk back commits for unused function parameters - they're still being
used via dragonegg for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208016 91177308-0d34-0410-b5e6-96231b3b80d8