Cleanup BB vectorization roots handling

The following moves CONSTRUCTOR handling into the generic BB
vectorization roots handling, removing a special case and finally
renaming the function now consisting of more than just constructor
detection.

	* tree-vect-slp.cc (vect_analyze_slp_instance): Remove
	slp_inst_kind_ctor handling.
	(vect_analyze_slp): Simplify.
	(vect_build_slp_instance): Dump when we analyze a CTOR.
	(vect_slp_check_for_constructors): Rename to ...
	(vect_slp_check_for_roots): ... this.  Register a
	slp_root for CONSTRUCTORs instead of shoving them to
	the set of grouped stores.
	(vect_slp_analyze_bb_1): Adjust.