Fall back to masked_gather_load/masked_scatter_store

This adds a fallback to the masked_ variants for gather_load
and scatter_store if the latter are not available.

2021-08-17  Richard Biener  <rguenther@suse.de>

	* optabs-query.c (supports_vec_gather_load_p): Also check
	for masked optabs.
	(supports_vec_scatter_store_p): Likewise.
	* tree-vect-data-refs.c (vect_gather_scatter_fn_p): Fall
	back to masked variants if non-masked are not supported.
	* tree-vect-patterns.c (vect_recog_gather_scatter_pattern):
	When we need to use masked gather/scatter but do not have
	a mask set up a constant true one.
	* tree-vect-stmts.c (vect_check_scalar_mask): Also allow
	non-SSA_NAME masks.