poly_int: expand_assignment

poly_int: expand_assignment

This patch makes the CONCAT handing in expand_assignment cope with
polynomial mode sizes.  The mode of the CONCAT must be complex,
so we can base the tests on the sizes of the real and imaginary
components.

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

gcc/
	* expr.c (expand_assignment): Cope with polynomial mode sizes
	when assigning to a CONCAT.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256199