tree-optimization/115528 - fix vect alignment analysis for outer loop vect

tree-optimization/115528 - fix vect alignment analysis for outer loop vect

For outer loop vectorization of a data reference in the inner loop
we have to look at both steps to see if they preserve alignment.

What is special for this testcase is that the outer loop step is
one element but the inner loop step four and that we now use SLP
and the vectorization factor is one.

	PR tree-optimization/115528
	* tree-vect-data-refs.cc (vect_compute_data_ref_alignment):
	Make sure to look at both the inner and outer loop step
	behavior.

	* gfortran.dg/vect/pr115528.f: New testcase.