Make SLP root stmt a vector

Make SLP root stmt a vector

This fixes a TODO noticed when adding vectorization of
BIT_INSERT_EXPRs and what's now useful for vectorization of
BB reductions.

2021-06-08  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (_slp_instance::root_stmt): Change to...
	(_slp_instance::root_stmts): ... a vector.
	(SLP_INSTANCE_ROOT_STMT): Rename to ...
	(SLP_INSTANCE_ROOT_STMTS): ... this.
	(slp_root::root): Change to...
	(slp_root::roots): ... a vector.
	(slp_root::slp_root): Adjust.
	* tree-vect-slp.c (_slp_instance::location): Adjust.
	(vect_free_slp_instance): Release the root stmt vector.
	(vect_build_slp_instance): Adjust.
	(vect_analyze_slp): Likewise.
	(_bb_vec_info::~_bb_vec_info): Likewise.
	(vect_slp_analyze_operations): Likewise.
	(vect_bb_vectorization_profitable_p): Likewise.  Adjust
	costs for the root stmt.
	(vect_slp_check_for_constructors): Gather all BIT_INSERT_EXPRs
	as root stmts.
	(vect_slp_analyze_bb_1): Simplify by marking all root stmts
	as pure_slp.
	(vectorize_slp_instance_root_stmt): Adjust.
	(vect_schedule_slp): Likewise.