get_best_mode always returns a scalar_int_mode on success,
so this patch makes that explicit in the type system.  Also,
the "largest_mode" argument is used simply to provide a maximum
size, and in practice that size is always a compile-time constant,
even when the concept of variable-sized modes is added later.
The patch therefore passes the size directly.

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* machmode.h (bit_field_mode_iterator::next_mode): Take a pointer
	to a scalar_int_mode instead of a machine_mode.
	(bit_field_mode_iterator::m_mode): Change type to opt_scalar_int_mode.
	(get_best_mode): Return a boolean and use a pointer argument to store
	the selected mode.  Replace the limit mode parameter with a bit limit.
	* expmed.c (adjust_bit_field_mem_for_reg): Use scalar_int_mode
	for the values returned by bit_field_mode_iterator::next_mode.
	(store_bit_field): Update call to get_best_mode.
	(store_fixed_bit_field): Likewise.
	(extract_fixed_bit_field): Likewise.
	* expr.c (optimize_bitfield_assignment_op): Likewise.
	* fold-const.c (optimize_bit_field_compare): Likewise.
	(fold_truth_andor_1): Likewise.
	* stor-layout.c (bit_field_mode_iterator::next_mode): As above.
	Update for new type of m_mode.
	(get_best_mode): As above.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r251491