Add -fslp-vectorize to enable bb-vectorize

Add -fslp-vectorize (with -ftree-slp-vectorize as an alias for gcc compatibility)
to provide a way to enable the basic-block vectorization pass. This uses the same
acronym as gcc, superword-level parallelism (SLP), also common in the literature,
to refer to basic-block vectorization.

Nadav suggested this as a follow-up to the adding of -fvectorize.

llvm-svn: 169909