poly_int: REGMODE_NATURAL_SIZE

This patch makes target-independent code that uses REGMODE_NATURAL_SIZE
treat it as a poly_int rather than a constant.

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

gcc/
	* combine.c (can_change_dest_mode): Handle polynomial
	REGMODE_NATURAL_SIZE.
	* expmed.c (store_bit_field_1): Likewise.
	* expr.c (store_constructor): Likewise.
	* emit-rtl.c (validate_subreg): Operate on polynomial mode sizes
	and polynomial REGMODE_NATURAL_SIZE.
	(gen_lowpart_common): Likewise.
	* reginfo.c (record_subregs_of_mode): Likewise.
	* rtlanal.c (read_modify_subreg_p): Likewise.

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

From-SVN: r256149