This fixes bogus misalignment calculation for negative steps
since an assertion a previous comment indicated no longer holds:

      /* DR_STEP(dr) is the same as -TYPE_SIZE of the scalar type,
         otherwise we wouldn't be here.  */

Thus the following replaces DR_STEP by -TYPE_SIZE.

2020-07-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96075
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment): Use
	TYPE_SIZE_UNIT of the vector component type instead of DR_STEP
	for the misalignment calculation for negative step.

	* gcc.dg/vect/slp-46.c: New testcase.

(cherry picked from commit dccbf1e2a6e544f71b4a5795f0c79015db019fc3)