Tighten early exit in vect_analyze_data_ref_dependence (PR85586)

Tighten early exit in vect_analyze_data_ref_dependence (PR85586)

The problem in this PR was that we didn't consider aliases between
writes in the same strided group.  After tightening the early exit
we get the expected abs(step) >= 2 versioning check.

2018-05-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/85586
	* tree-vect-data-refs.c (vect_analyze_data_ref_dependence): Only
	exit early for statements in the same group if the accesses are
	not strided.

gcc/testsuite/
	PR tree-optimization/85586
	* gcc.dg/vect/pr85586.c: New test.

From-SVN: r259822