tree-optimization/96783 - fix vectorization of negative step SLP

tree-optimization/96783 - fix vectorization of negative step SLP

This appropriately uses VMAT_ELEMENTWISE when the vectors cannot be
filled from a single SLP group until we get more explicit support
for negative stride SLP.

2020-08-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96783
	* tree-vect-stmts.c (get_group_load_store_type): Use
	VMAT_ELEMENTWISE for negative strides when we cannot
	use VMAT_STRIDED_SLP.

	* gcc.dg/vect/pr96783-1.c: New testcase.
	* gcc.dg/vect/pr96783-2.c: Likewise.