Add support for adjusting the number of units in a mode

Add support for adjusting the number of units in a mode

We already allow the target to change the size and alignment of a mode.
This patch does the same thing for the number of units, which is needed
to give command-line control of the SVE vector length.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* machmode.h (mode_precision): Prefix with CONST_MODE_PRECISION.
	(mode_nunits): Likewise CONST_MODE_NUNITS.
	* machmode.def (ADJUST_NUNITS): Document.
	* genmodes.c (mode_data::need_nunits_adj): New field.
	(blank_mode): Update accordingly.
	(adj_nunits): New variable.
	(print_maybe_const_decl): Replace CATEGORY with a NEEDS_ADJ
	parameter.
	(emit_mode_size_inline): Set need_bytesize_adj for all modes
	listed in adj_nunits.
	(emit_mode_nunits_inline): Set need_nunits_adj for all modes
	listed in adj_nunits.  Don't emit case statements for such modes.
	(emit_insn_modes_h): Emit definitions of CONST_MODE_NUNITS
	and CONST_MODE_PRECISION.  Make CONST_MODE_SIZE expand to
	nothing if adj_nunits is nonnull.
	(emit_mode_precision, emit_mode_nunits): Use print_maybe_const_decl.
	(emit_mode_unit_size, emit_mode_base_align, emit_mode_ibit)
	(emit_mode_fbit): Update use of print_maybe_const_decl.
	(emit_move_size): Likewise.  Treat the array as non-const
	if adj_nunits.
	(emit_mode_adjustments): Handle adj_nunits.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256204