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.