poly_int: expand_ifn_atomic_compare_exchange_into_call

This patch makes the mode size assumptions in
expand_ifn_atomic_compare_exchange_into_call a bit more
explicit, so that a later patch can add a to_constant () call.

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

gcc/
	* builtins.c (expand_ifn_atomic_compare_exchange_into_call): Assert
	that the mode size is in the set {1, 2, 4, 8, 16}.

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

From-SVN: r256157