Add an option -vectorize-slp-aggressive for running the BB vectorizer. Make -fslp-vectorize run the slp-vectorizer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179508 91177308-0d34-0410-b5e6-96231b3b80d8
Docs: merge the description of the BB and SLP vectorizers and document the -fslp-vectorize-aggressive flag.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179510 91177308-0d34-0410-b5e6-96231b3b80d8
Document our desire to enable the loop vectorizer on -Os in future releases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179511 91177308-0d34-0410-b5e6-96231b3b80d8