re PR rtl-optimization/81308 (ICE in calc_dfs_tree, at dominance.c:458)

re PR rtl-optimization/81308 (ICE in calc_dfs_tree, at dominance.c:458)

	PR rtl-optimizatin/81308
	* tree-switch-conversion.c (cfg_altered): New file scoped static.
	(process_switch): If group_case_labels makes a change, then set
	cfg_altered.
	(pass_convert_switch::execute): If a switch is converted, then
	set cfg_altered.  Return TODO_cfg_cleanup if cfg_altered is true.

	PR rtl-optimizatin/81308
	* g++.dg/pr81308-1.C: New test.
	* g++.dg/pr81308-2.C: New test.

From-SVN: r256349