vect: Add extraction cost for slp reduc

As Richi pointed out, currently for BB reductions we don't
actually build a SLP node with IFN_REDUC_* information,
ideally we may end up with that eventually.

For now, it's costed as shuffles and reduc operations, it
misses the cost of lane extraction.  This patch is to add
one time of vec_to_scalar cost for lane extraction, it's to
make the costings consistent and conservative for now.

gcc/ChangeLog:

	* tree-vect-slp.c (vectorizable_bb_reduc_epilogue): Add the cost for
	value extraction.