refactor SLP analysis

This passes down the graph entry kind down to vect_analyze_slp_instance
which simplifies it and makes it a shallow wrapper around
vect_build_slp_instance.

2020-11-06  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_analyze_slp): Pass down the
	SLP graph entry kind.
	(vect_analyze_slp_instance): Simplify.
	(vect_build_slp_instance): Adjust.
	(vect_slp_check_for_constructors): Perform more
	eligibility checks here.