poly_int: set_inc_state

This trivial patch makes auto-inc-dec.c:set_inc_state take a poly_int64.

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

gcc/
	* auto-inc-dec.c (set_inc_state): Take the mode size as a poly_int64
	rather than an int.

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

From-SVN: r256153