This patch makes vect_compute_data_ref_alignment treat DR_INIT as a
poly_int and handles cases in which the calculated misalignment might
not be constant.

2017-12-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment):
	Treat drb->init as a poly_int.  Fail if its misalignment wrt
	vector_alignment isn't known.

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

From-SVN: r255935