fix detection of negative step DR groups

fix detection of negative step DR groups

This fixes a condition that caused all negative step DR groups to
be detected as single element interleaving.  Such groups are
rejected by interleaving vectorization but miscompiled by SLP
which is fixed by forcing VMAT_STRIDED_SLP for now.

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

	* tree-vect-data-refs.c (vect_analyze_data_ref_accesses): Fix
	group overlap condition to allow negative step DR groups.
	* tree-vect-stmts.c (get_group_load_store_type): For
	multi element SLP groups force VMAT_STRIDED_SLP when the step
	is negative.

	* gcc.dg/vect/slp-47.c: New testcase.
	* gcc.dg/vect/slp-48.c: Likewise.