[67/77] Use scalar_mode in fixed-value.*

This patch makes the fixed-value.* routines use scalar_mode.
It would be possible to define special classes for these modes, as for
scalar_int_mode and scalar_float_mode, but at the moment nothing would
benefit from them.  In particular, there's no use case that would help
select between one class for all fixed-point modes versus one class for
fractional modes and one class for accumulator modes.

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

gcc/
	* fixed-value.h (fixed_from_double_int): Take a scalar_mode
	rather than a machine_mode.
	(fixed_from_string): Likewise.
	(fixed_convert): Likewise.
	(fixed_convert_from_int): Likewise.
	(fixed_convert_from_real): Likewise.
	(real_convert_from_fixed): Likewise.
	* fixed-value.c (fixed_from_double_int): Likewise.
	(fixed_from_string): Likewise.
	(fixed_convert): Likewise.
	(fixed_convert_from_int): Likewise.
	(fixed_convert_from_real): Likewise.
	(real_convert_from_fixed): Likewise.
	* config/avr/avr.c (avr_out_round): Use as_a <scalar_mode>.

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

From-SVN: r251518