Add explicit SLP_TREE_LANES

Add explicit SLP_TREE_LANES

This adds an explicit number of scalar lanes to the SLP node
avoiding to dispatch between stmts/ops and eventually not require
those vectors at all.

2020-05-27  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (_slp_tree::lanes): New.
	(SLP_TREE_LANES): Likewise.
	* tree-vect-loop.c (vect_create_epilog_for_reduction): Use it.
	(vectorizable_reduction): Likewise.
	(vect_transform_cycle_phi): Likewise.
	(vectorizable_induction): Likewise.
	(vectorizable_live_operation): Likewise.
	* tree-vect-slp.c (_slp_tree::_slp_tree): Initialize lanes.
	(vect_create_new_slp_node): Likewise.
	(slp_copy_subtree): Copy it.
	(vect_optimize_slp): Use it.
	(vect_slp_analyze_node_operations_1): Likewise.
	(vect_slp_convert_to_external): Likewise.
	(vect_bb_vectorization_profitable_p): Likewise.
	* tree-vect-stmts.c (vectorizable_load): Likewise.
	(get_vectype_for_scalar_type): Likewise.