Make mode_for_size_tree return an opt_mode

Make mode_for_size_tree return an opt_mode

...for consistency with mode_for_size

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

gcc/
	* stor-layout.h (mode_for_size_tree): Return an opt_mode.
	* stor-layout.c (mode_for_size_tree): Likewise.
	(mode_for_array): Update accordingly.
	(layout_decl): Likewise.
	(compute_record_mode): Likewise.  Only set the mode once.

gcc/ada/
	* gcc-interface/utils.c (make_packable_type): Update call to
	mode_for_size_tree.

From-SVN: r251732