This PR is another regression caused by the removal of the simple_iv
check in dr_analyze_innermost for BB analysis.  Without splitting out
the step, we weren't able to find an underlying object whose alignment
could be increased.

As with PR81635, I think the simple_iv was only handling one special
case of something that ought to be more general.  The more general
thing here is that if the address can be analysed as a scalar
evolution, and if all updates preserve alignment N, it's possible
to align the address to N by increasing the alignment of the base
object to N.  That applies also to outer loops, and to both loop
and BB analysis.

I wasn't sure where the new functions ought to live, but tree-data-ref.c
seemed OK since (a) that already does scev analysis on addresses and
(b) you'd want to use dr_analyze_innermost first if you were analysing
a reference.

2018-03-24  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/84005
	* tree-data-ref.h (get_base_for_alignment): Declare.
	* tree-data-ref.c (get_base_for_alignment_1): New function.
	(get_base_for_alignment): Likewise.
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment): Use
	get_base_for_alignment to find a suitable base object, instead
	of always using drb->base_address.

gcc/testsuite/
	PR tree-optimization/84005
	* gcc.dg/vect/bb-slp-1.c: Make sure there is no message about
	failing to force the alignment.

From-SVN: r258833