poly_int: process_alt_operands

This patch makes process_alt_operands check that the mode sizes
are ordered, so that match_reload can validly treat them as subregs
of one another.

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

gcc/
	* lra-constraints.c (process_alt_operands): Reject matched
	operands whose sizes aren't ordered.
	(match_reload): Refer to this check here.

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

From-SVN: r256158