Remove vect_get_vec_defs_for_operand

Remove vect_get_vec_defs_for_operand

This removes vect_get_vec_defs_for_operand and its remaining uses.
It also removes some remaining non-SLP paths in preparation to
elide STMT_VINFO_VEC_STMTS.

	* tree-vectorizer.h (vect_get_vec_defs_for_operand): Remove.
	* tree-vect-stmts.cc (vect_get_vec_defs_for_operand): Likewise.
	(vect_get_vec_defs): Remove non-SLP path.
	(check_load_store_for_partial_vectors): We always have an
	SLP node.
	(vect_check_store_rhs): Likewise.
	(vect_get_gather_scatter_ops): Likewise.
	(vect_create_vectorized_demotion_stmts): Likewise.
	(vectorizable_store): Adjust.
	(vectorizable_load): Likewise.