gcc compilation broken with --enable-checking=release

[Jan Hubicka]

Hi,
I've committed the following fix.

	* cgraph.h (struct cgraph_node): Mark former_clone_of by GTY ((skip)).
	* cgraphunit.c (clone_of_p): Compile only when checking is enabled.
Index: cgraph.h
===================================================================
*** cgraph.h	(revision 159923)
--- cgraph.h	(working copy)
*************** struct GTY((chain_next ("%h.next"), chai
*** 227,234 ****
       to the edge to speed up cgraph_edge function.  */
    htab_t GTY((param_is (struct cgraph_edge))) call_site_hash;
  #ifdef ENABLE_CHECKING
!   /* Declaration node used to be clone of.  Used for checking only.  */
!   tree former_clone_of;
  #endif
  
    PTR GTY ((skip)) aux;
--- 227,235 ----
       to the edge to speed up cgraph_edge function.  */
    htab_t GTY((param_is (struct cgraph_edge))) call_site_hash;
  #ifdef ENABLE_CHECKING
!   /* Declaration node used to be clone of.  Used for checking only. 
!      We must skip it or we get references from release checking GGC files. */
!   tree GTY ((skip)) former_clone_of;
  #endif
  
    PTR GTY ((skip)) aux;
Index: cgraphunit.c
===================================================================
*** cgraphunit.c	(revision 159923)
--- cgraphunit.c	(working copy)
*************** cgraph_mark_if_needed (tree decl)
*** 557,562 ****
--- 557,563 ----
      cgraph_mark_needed_node (node);
  }
  
+ #ifdef ENABLE_CHECKING
  /* Return TRUE if NODE2 is equivalent to NODE or its clone.  */
  static bool
  clone_of_p (struct cgraph_node *node, struct cgraph_node *node2)
*************** clone_of_p (struct cgraph_node *node, st
*** 565,570 ****
--- 566,572 ----
      node2 = node2->clone_of;
    return node2 != NULL;
  }
+ #endif
  
  /* Verify cgraph nodes of given cgraph node.  */
  void