Always use SLP vector defs to determine insertion place

Always use SLP vector defs to determine insertion place

With the last vectorizable_shift patch we can now always use the
SLP vector defs to determine the vectorized stmt insertion place,
paving the way for a "verifier" for pending restructuring and
BB vectorization of reductions and other live stmts.

2020-06-25  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_schedule_slp_instance): Always use
	vector defs to determine insertion place.