Use VEC_PERM_EXPR in the vectorizer.

Use VEC_PERM_EXPR in the vectorizer.

        * tree-vect-slp.c: Include langhooks.h.
        (vect_create_mask_and_perm): Emit VEC_PERM_EXPR, not a builtin.
        (vect_transform_slp_perm_load): Use can_vec_perm_expr_p.  Simplify
        mask creation for VEC_PERM_EXPR.
        * tree-vect-stmts.c (perm_mask_for_reverse): Return the mask,
        not the builtin.
        (reverse_vec_elements): Emit VEC_PERM_EXPR not a builtin.
        * Makefile.in (tree-vect-slp.o): Update dependency.
        * optabs.c (can_vec_perm_expr_p): Allow NULL as unknown constant.

From-SVN: r180047