Remove Clang support for '-fvectorize-slp-aggressive' which used LLVM's

basic block vectorizer. This vectorizer has had no known users for many,
many years and is completely surpassed by the normal
'-fvectorize-slp'-controlled SLP vectorizer in LLVM.

Hal proposed this back in 2014 to no objections:
http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html

While this patch completely removes the flag, Joerg is working on
a patch that will add it back in a way that warns users and ignores the
flag in a clear and well factored way (so that we can keep doing this
going forward).

Differential Revision: https://reviews.llvm.org/D34846

llvm-svn: 306786