tree-optimization/116647 - wrong classified double reduction

The following avoids classifying a double reduction that's not
actually a reduction in the outer loop (because its value isn't
used outside of the outer loop).  This avoids us ICEing on the
unexpected stmt/SLP node arrangement.

	PR tree-optimization/116647
	* tree-vect-loop.cc (vect_is_simple_reduction): Add missing
	check to double reduction detection.

	* gcc.dg/torture/pr116647.c: New testcase.
	* gcc.dg/vect/no-scevccp-pr86725-2.c: Adjust expected pattern.
	* gcc.dg/vect/no-scevccp-pr86725-4.c: Likewise.