[71/77] Use opt_scalar_mode for mode iterators

This patch uses opt_scalar_mode when iterating over scalar modes.

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

gcc/
	* coretypes.h (opt_scalar_mode): New typedef.
	* gdbhooks.py (build_pretty_printers): Handle it.
	* machmode.h (mode_iterator::get_2xwider): Add overload for
	opt_mode<T>.
	* emit-rtl.c (init_emit_once): Use opt_scalar_mode when iterating
	over scalar modes.
	* expr.c (convert_mode_scalar): Likewise.
	* omp-low.c (omp_clause_aligned_alignment): Likewise.
	* optabs.c (expand_float): Likewise.
	(expand_fix): Likewise.
	* tree-vect-stmts.c (vectorizable_conversion): Likewise.

gcc/c-family/
	* c-common.c (c_common_fixed_point_type_for_size): Use opt_scalar_mode
	for the mode iterator.

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

From-SVN: r251522