This patch passes the data vector mode to get_mask_mode, rather than its
size and nunits.  This is a bit simpler and allows targets to distinguish
between modes that happen to have the same size and number of elements.

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* target.def (get_mask_mode): Take a vector mode itself as argument,
	instead of properties about the vector mode.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_get_mask_mode): Update to reflect new
	get_mode_mask interface.
	* targhooks.c (default_get_mask_mode): Likewise.  Use
	related_int_vector_mode.
	* optabs-query.c (can_vec_mask_load_store_p): Update call
	to get_mask_mode.
	* tree-vect-stmts.c (check_load_store_masking): Likewise, checking
	first that the original mode really is a vector.
	* tree.c (build_truth_vector_type_for): Likewise.
	* config/aarch64/aarch64.c (aarch64_get_mask_mode): Update for new
	get_mode_mask interface.
	(aarch64_expand_sve_vcond): Update call accordingly.
	* config/gcn/gcn.c (gcn_vectorize_get_mask_mode): Update for new
	get_mode_mask interface.
	* config/i386/i386.c (ix86_get_mask_mode): Likewise.

From-SVN: r278233