tree-optimization/97760 - reduction paths with unhandled live stmt

tree-optimization/97760 - reduction paths with unhandled live stmt

This makes sure we reject reduction paths with a live stmt that
is not the last one altering the value.  This is because we do not
handle this in the epilogue unless there's a scalar epilogue loop.

2020-11-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97760
	* tree-vect-loop.c (check_reduction_path): Reject
	reduction paths we do not handle in epilogue generation.

	* gcc.dg/vect/pr97760.c: New testcase.