Remove the BBVectorize pass.

It served us well, helped kick-start much of the vectorization efforts
in LLVM, etc. Its time has come and past. Back in 2014:
http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html

Time to actually let go and move forward. =]

I've updated the release notes both about the removal and the
deprecation of the corresponding C API.

llvm-svn: 306797