poly_int: reloading complex subregs

This patch splits out a condition that is common to both push_reload
and reload_inner_reg_of_subreg.

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

gcc/
	* reload.c (complex_word_subreg_p): New function.
	(reload_inner_reg_of_subreg, push_reload): Use it.

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

From-SVN: r256159