Update the docs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187713 91177308-0d34-0410-b5e6-96231b3b80d8
Update the release notes about the status of the vectorizers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187714 91177308-0d34-0410-b5e6-96231b3b80d8