rtl-optimization/117467 - 33% compile-time in rest of compilation

ext-dce uses TV_NONE, that's not OK for a pass taking 33% compile-time.
The following adds a timevar to it for proper blaming.

	PR rtl-optimization/117467
	* timevar.def (TV_EXT_DCE): New.
	* ext-dce.cc (pass_data_ext_dce): Use TV_EXT_DCE.

gcc/ext-dce.cc

@@ -1103,7 +1103,7 @@ const pass_data pass_data_ext_dce =
   RTL_PASS, /* type */
   "ext_dce", /* name */
   OPTGROUP_NONE, /* optinfo_flags */
-  TV_NONE, /* tv_id */
+  TV_EXT_DCE, /* tv_id */
   PROP_cfglayout, /* properties_required */
   0, /* properties_provided */
   0, /* properties_destroyed */

gcc/timevar.def

@@ -313,6 +313,7 @@ DEFTIMEVAR (TV_INITIALIZE_RTL        , "initialize rtl")
 DEFTIMEVAR (TV_GIMPLE_LADDRESS       , "address lowering")
 DEFTIMEVAR (TV_TREE_LOOP_IFCVT       , "tree loop if-conversion")
 DEFTIMEVAR (TV_WARN_ACCESS           , "access analysis")
+DEFTIMEVAR (TV_EXT_DCE               , "ext dce")
 
 /* Everything else in rest_of_compilation not included above.  */
 DEFTIMEVAR (TV_EARLY_LOCAL	     , "early local passes")