Use make_skip to turn an instruction into a SKIP

Author: tautschnig

src/analyses/goto_check.cpp

@@ -1649,12 +1649,16 @@ void goto_checkt::goto_check(
       if((is_user_provided && !enable_assertions &&
           i.source_location.get_property_class()!="error label") ||
          (!is_user_provided && !enable_built_in_assertions))
-        i.type=SKIP;
+      {
+        i.make_skip();
+      }
     }
     else if(i.is_assume())
     {
       if(!enable_assumptions)
-        i.type=SKIP;
+      {
+        i.make_skip();
+      }
     }
     else if(i.is_dead())
     {

src/goto-programs/set_properties.cpp

@@ -32,7 +32,7 @@ void set_properties(
       property_set.find(property_id);
 
     if(c_it==property_set.end())
-      it->type=SKIP;
+      it->make_skip();
     else
       property_set.erase(c_it);
   }