Make targetm.get_mask_mode return an opt_mode

Make targetm.get_mask_mode return an opt_mode

...for consistency with mode_for_vector.

2017-09-05  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* target.def (get_mask_mode): Change return type to opt_mode.
	Expand commentary.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_get_mask_mode): Return an opt_mode.
	* targhooks.c (default_get_mask_mode): Likewise.
	* config/i386/i386.c (ix86_get_mask_mode): Likewise.
	* optabs-query.c (can_vec_mask_load_store_p): Update use of
	targetm.get_mask_mode.
	* tree.c (build_truth_vector_type): Likewise.

From-SVN: r251731