ira.c bb_loop_depth again

Follow the same practice as other places in ira.c, where
free_dominance_info is called along with loop_optimizer_finalize.  Not
doing so causes an ICE on gcc-5-branch, so avoid that possibility on
trunk.

	PR rtl-optimization/71275
	* ira.c (ira): Free dominance info.

From-SVN: r236843

gcc/ChangeLog

+2016-05-28  Alan Modra  <amodra@gmail.com>
+
+	PR rtl-optimization/71275
+	* ira.c (ira): Free dominance info.
+
 2016-05-27  Gerald Pfeifer  <gerald@pfeifer.com>
 
 	* doc/sourcebuild.texi: New address for upstream Go repository.

gcc/ira.c

@@ -5188,6 +5188,7 @@ ira (FILE *f)
     add_store_equivs ();
 
   loop_optimizer_finalize ();
+  free_dominance_info (CDI_DOMINATORS);
   end_alias_analysis ();
   free (reg_equiv);