Remove SLP_TREE_VEC_STMTS in favor of SLP_TREE_VEC_DEFS

Remove SLP_TREE_VEC_STMTS in favor of SLP_TREE_VEC_DEFS

The following unifies SLP_TREE_VEC_STMTS into SLP_TREE_VEC_DEFS
which can handle all cases we need.

	* tree-vectorizer.h (_slp_tree::push_vec_def): Add.
	(_slp_tree::vec_stmts): Remove.
	(SLP_TREE_VEC_STMTS): Remove.
	* tree-vect-slp.cc (_slp_tree::push_vec_def): Define.
	(_slp_tree::_slp_tree): Adjust.
	(_slp_tree::~_slp_tree): Likewise.
	(vect_get_slp_vect_def): Simplify.
	(vect_get_slp_defs): Likewise.
	(vect_transform_slp_perm_load_1): Adjust.
	(vect_add_slp_permutation): Likewise.
	(vect_schedule_slp_node): Likewise.
	(vectorize_slp_instance_root_stmt): Likewise.
	(vect_schedule_scc): Likewise.
	* tree-vect-stmts.cc (vectorizable_bswap): Use push_vec_def.
	(vectorizable_call): Likewise.
	(vectorizable_call): Likewise.
	(vect_create_vectorized_demotion_stmts): Likewise.
	(vectorizable_conversion): Likewise.
	(vectorizable_assignment): Likewise.
	(vectorizable_shift): Likewise.
	(vectorizable_operation): Likewise.
	(vectorizable_load): Likewise.
	(vectorizable_condition): Likewise.
	(vectorizable_comparison): Likewise.
	* tree-vect-loop.cc (vect_create_epilog_for_reduction): Adjust.
	(vectorize_fold_left_reduction): Use push_vec_def.
	(vect_transform_reduction): Likewise.
	(vect_transform_cycle_phi): Likewise.
	(vectorizable_lc_phi): Likewise.
	(vectorizable_phi): Likewise.
	(vectorizable_recurr): Likewise.
	(vectorizable_induction): Likewise.
	(vectorizable_live_operation): Likewise.