vect: Fix offset calculation for -ve strides [PR93767]

vect: Fix offset calculation for -ve strides [PR93767]

This PR is a regression caused by r256644, which added support for alias
checks involving variable strides.  One of the changes in that commit
was to split the access size out of the segment length.  The PR shows
that I hadn't done that correctly for the handling of negative strides
in vect_compile_time_alias.  The old code was:

      const_length_a = (-wi::to_poly_wide (segment_length_a)).force_uhwi ();
      offset_a = (offset_a + vect_get_scalar_dr_size (a)) - const_length_a;

where vect_get_scalar_dr_size (a) was cancelling out the subtraction
of the access size inherent in "- const_length_a".  Taking the access
size out of the segment length meant that the addition was no longer
needed/correct.

2020-02-19  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/93767
	* tree-vect-data-refs.c (vect_compile_time_alias): Remove the
	access-size bias from the offset calculations for negative strides.

gcc/testsuite/
	PR tree-optimization/93767
	* gcc.dg/vect/pr93767.c: New test.