Document the loop vectorizer changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174585 91177308-0d34-0410-b5e6-96231b3b80d8
Update Release notes regarding TTI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174586 91177308-0d34-0410-b5e6-96231b3b80d8
[ReleaseNotes] tidy up organization and formatting

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174587 91177308-0d34-0410-b5e6-96231b3b80d8