[11/77] Add a float_mode_for_size helper function

[11/77] Add a float_mode_for_size helper function

This provides a type-safe way to ask for a float mode and get it as a
scalar_float_mode.

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_float_mode): New typedef.
	* machmode.h (float_mode_for_size): New function.
	* emit-rtl.c (double_mode): Delete.
	(init_emit_once): Use float_mode_for_size.
	* stor-layout.c (layout_type): Likewise.
	* gdbhooks.py (build_pretty_printer): Handle opt_scalar_float_mode.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251463