The following delays adjusting the SLP graph for converted reduction
chains to a point where the SLP build no longer can fail since we
otherwise fail to undo marking the conversion as a group.

2020-01-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93397
	* tree-vect-slp.c (vect_analyze_slp_instance): Delay
	converted reduction chain SLP graph adjustment.

	* gcc.dg/torture/pr93397.c: New testcase.