Add a fixed_size_mode class

Add a fixed_size_mode class

This patch adds a fixed_size_mode machine_mode wrapper
for modes that are known to have a fixed size.  That applies
to all current modes, but future patches will add support for
variable-sized modes.

The use of this class should be pretty restricted.  One important
use case is to hold the mode of static data, which can never be
variable-sized with current file formats.  Another is to hold
the modes of registers involved in __builtin_apply and
__builtin_result, since those interfaces don't cope well with
variable-sized data.

The class can also be useful when reinterpreting the contents of
a fixed-length bit string as a different kind of value.

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* machmode.h (fixed_size_mode): New class.
	* rtl.h (get_pool_mode): Return fixed_size_mode.
	* gengtype.c (main): Add fixed_size_mode.
	* target.def (get_raw_result_mode): Return a fixed_size_mode.
	(get_raw_arg_mode): Likewise.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_get_reg_raw_mode): Return a fixed_size_mode.
	* targhooks.c (default_get_reg_raw_mode): Likewise.
	* config/ia64/ia64.c (ia64_get_reg_raw_mode): Likewise.
	* config/mips/mips.c (mips_get_reg_raw_mode): Likewise.
	* config/msp430/msp430.c (msp430_get_raw_arg_mode): Likewise.
	(msp430_get_raw_result_mode): Likewise.
	* config/avr/avr-protos.h (regmask): Use as_a <fixed_side_mode>
	* dbxout.c (dbxout_parms): Require fixed-size modes.
	* expr.c (copy_blkmode_from_reg, copy_blkmode_to_reg): Likewise.
	* gimple-ssa-store-merging.c (encode_tree_to_bitpos): Likewise.
	* omp-low.c (lower_oacc_reductions): Likewise.
	* simplify-rtx.c (simplify_immed_subreg): Take fixed_size_modes.
	(simplify_subreg): Update accordingly.
	* varasm.c (constant_descriptor_rtx::mode): Change to fixed_size_mode.
	(force_const_mem): Update accordingly.  Return NULL_RTX for modes
	that aren't fixed-size.
	(get_pool_mode): Return a fixed_size_mode.
	(output_constant_pool_2): Take a fixed_size_mode.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254300 138bc75d-0d04-0410-961f-82ee72b054a4