Add a helper for getting the overall alignment of a DR

Add a helper for getting the overall alignment of a DR

This combines the information from previous patches to give a guaranteed
alignment for the DR as a whole.  This should be a bit safer than using
base_element_aligned, since that only really took the base into account
(not the init or offset).

2017-07-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-data-ref.h (dr_alignment): Declare.
	* tree-data-ref.c (dr_alignment): New function.
	* tree-vectorizer.h (dataref_aux): Remove base_element_aligned.
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment): Don't
	set it.
	* tree-vect-stmts.c (vectorizable_store): Use dr_alignment.

From-SVN: r249917