+ * Allocate a frag array for a node if the current graph state is phase_building.
+ *
+ * @param irn the node for which the frag array should be allocated
+ * @param op the opcode of the (original) node, if does not match opcode of irn,
+ * nothing is done
+ * @param frag_store the address of the frag store in irn attributes, if this
+ * address contains a value != NULL, does nothing