a const ir_graph is enough for get_idx_irn

include/libfirm/irgraph.h

@@ -259,7 +259,7 @@ FIRM_API size_t get_irg_idx(const ir_graph *irg);
  *            index.
  * @note      The node you got might be dead.
  */
-FIRM_API ir_node *get_idx_irn(ir_graph *irg, unsigned idx);
+FIRM_API ir_node *get_idx_irn(const ir_graph *irg, unsigned idx);
 
 
 /******************************************************************************/

ir/ir/irgraph.c

@@ -491,7 +491,7 @@ size_t get_irg_idx(const ir_graph *irg)
 	return irg->index;
 }
 
-ir_node *(get_idx_irn)(ir_graph *irg, unsigned idx)
+ir_node *(get_idx_irn)(const ir_graph *irg, unsigned idx)
 {
 	return _get_idx_irn(irg, idx);
 }

ir/ir/irgraph_t.h

@@ -447,7 +447,7 @@ static inline void irg_kill_node(ir_graph *irg, ir_node *n)
  * @return    The node with that index or NULL, if there is no node with that index.
  * @note      The node you got might be dead.
  */
-static inline ir_node *_get_idx_irn(ir_graph *irg, unsigned idx)
+static inline ir_node *_get_idx_irn(const ir_graph *irg, unsigned idx)
 {
 	assert(idx < (unsigned) ARR_LEN(irg->idx_irn_map));
 	return irg->idx_irn_map[idx];