Create a load/store vectorization pass.
Create a load/store vectorization pass.

This is a very basic pass that works for the vector loops we
generate. Emitting vector code directly would be nicer, but
that can't be done reasonably without a progressive lowering.

Once this pass is enabled, we will no longer rely on unrolling
and LLVM's autovectorization, allowing us to vectorize more
loads and stores (essentially, we can ditch the
MayPreventVectorization heuristic).
