[76/77] Add a scalar_mode_pod class

This patch adds a scalar_mode_pod class and uses it to
replace the machine_mode in fixed_value.

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

gcc/
	* coretypes.h (scalar_mode_pod): New typedef.
	* gdbhooks.py (build_pretty_printer): Handle it.
	* machmode.h (gt_ggc_mx, gt_pch_nx): New functions.
	* fixed-value.h (fixed_value::mode): Change type to scalar_mode_pod.
	* fold-const.c (fold_convert_const_int_from_fixed): Use scalar_mode.
	* tree-streamer-in.c (unpack_ts_fixed_cst_value_fields): 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: r251526