Remove group_size argument from vect_built_slp_tree* APIs

The group_size argument is redundant with the stmt vector length.

	* tree-vect-slp.cc (vect_build_slp_tree_1): Remove group_size
	argument and adjust.
	(vect_build_slp_tree_2): Likewise.
	(vect_build_slp_tree): Likewise.
	(optimize_load_redistribution_1): Likewise.
	(vect_build_slp_instance): Likewise.
	(vect_analyze_slp_reduc_chain): Likewise.
	(vect_analyze_slp_reduction): Likewise.
	(vect_analyze_slp_reduction_group): Likewise.
	(vect_analyze_slp_instance): Likewise.
	(vect_lower_load_permutations): Likewise.