[Vectorizer] Split vect_gen_perm_mask into _checked and _any variants

	* tree-vectorizer.h (vect_gen_perm_mask): Remove.
	(vect_gen_perm_mask_checked, vect_gen_perm_mask_any): New.

	tree_vec_data_refs.c (vect_permute_load_chain, vec_permute_store_chain,
	vec_shift_permute_load_chain): Replace vect_gen_perm_mask & assert
	with vect_gen_perm_mask_checked.

	* tree-vect-stmts.c (vectorizable_mask_load_store, vectorizable_load):
	Likewise.

	(vect_gen_perm_mask_checked): New.
	(vect_gen_perm_mask): Remove can_vec_perm_p check, rename to...
	(vect_gen_perm_mask_any): ...this.

	(perm_mask_for_reverse): Call can_vec_perm_p and
	vect_gen_perm_mask_checked.

From-SVN: r217508