Make mode_for_size return an opt_mode

Make mode_for_size return an opt_mode

...to make it consistent with int_mode_for_size etc.

require () seems like the right choice in replace_reg_with_saved_mem
because we use the chosen mode for saving and restoring registers,
which cannot be done in BLKmode.  Similarly require () seems like
the right choice in calls related to secondary memory reloads (the ones
in config/, and in get_secondary_mem) because the reload must always
have a defined mode, which e.g. determines the size of the slot.

We can use require () in simplify_subreg_concatn and assemble_integer
because it isn't meaningful to create a subreg with BLKmode (for one
thing, we couldn't tell then whether it was partial, paradoxical, etc.).

make_fract_type and make_accum_type must find a mode because that's
what distinguishes accumulator FIXED_POINT_TYPEs from fractional
FIXED_POINT_TYPEs.

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

gcc/
	* machmode.h (opt_machine_mode): New type.
	(opt_mode<T>): Allow construction from anything that can be
	converted to a T.
	(is_a, as_a, dyn_cast): Add overloads for opt_mode.
	(mode_for_size): Return an opt_machine_mode.
	* stor-layout.c (mode_for_size): Likewise.
	(mode_for_size_tree): Update call accordingly.
	(bitwise_mode_for_mode): Likewise.
	(make_fract_type): Likewise.
	(make_accum_type): Likewise.
	* caller-save.c (replace_reg_with_saved_mem): Update call
	accordingly.
	* config/alpha/alpha.h (SECONDARY_MEMORY_NEEDED_MODE): Likewise.
	* config/i386/i386.h (SECONDARY_MEMORY_NEEDED_MODE): Likewise.
	* config/s390/s390.h (SECONDARY_MEMORY_NEEDED_MODE): Likewise.
	* config/sparc/sparc.h (SECONDARY_MEMORY_NEEDED_MODE): Likewise.
	* expmed.c (extract_bit_field_1): Likewise.
	* reload.c (get_secondary_mem): Likewise.
	* varasm.c (assemble_integer): Likewise.
	* lower-subreg.c (simplify_subreg_concatn): Likewise.  Move
	early-out.

From-SVN: r251728