poly_int: get_object_alignment_2

poly_int: get_object_alignment_2

This patch makes get_object_alignment_2 track polynomial offsets
and sizes.  The real work is done by get_inner_reference, but we
then need to handle the alignment correctly.

2017-12-20  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* builtins.c (get_object_alignment_2): Track polynomial offsets
	and sizes.  Update the alignment handling.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r255893