Make mode_for_vector return an opt_mode

Make mode_for_vector return an opt_mode

...following on from the mode_for_size change.  The patch also removes
machmode.h versions of the stor-layout.c comments, since the comments
in the .c file are more complete.

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

gcc/
	* machmode.h (mode_for_vector): Return an opt_mode.
	* stor-layout.c (mode_for_vector): Likewise.
	(mode_for_int_vector): Update accordingly.
	(layout_type): Likewise.
	* config/i386/i386.c (emit_memmov): Likewise.
	(ix86_expand_set_or_movmem): Likewise.
	(ix86_expand_vector_init): Likewise.
	(ix86_get_mask_mode): Likewise.
	* config/powerpcspe/powerpcspe.c (rs6000_expand_vec_perm_const_1):
	Likewise.
	* config/rs6000/rs6000.c (rs6000_expand_vec_perm_const_1): Likewise.
	* expmed.c (extract_bit_field_1): Likewise.
	* expr.c (expand_expr_real_2): Likewise.
	* optabs-query.c (can_vec_perm_p): Likewise.
	(can_vec_mask_load_store_p): Likewise.
	* optabs.c (expand_vec_perm): Likewise.
	* targhooks.c (default_get_mask_mode): Likewise.
	* tree-vect-stmts.c (vectorizable_store): Likewise.
	(vectorizable_load): Likewise.
	(get_vectype_for_scalar_type_and_size): Likewise.

From-SVN: r251730