Change variable name

`mod` doesn't make any sense now that the `modified` flag has been
inverted and renamed `is_initializer`, so rename it to
`evs_non_initializer`

Author: Owen Jones

src/pointer-analysis/local_value_set.cpp

@@ -342,9 +342,9 @@ void local_value_sett::assign(
       const exprt &objexpr=object_numbering[obj.first];
       if(objexpr.id()==ID_external_value_set)
       {
-        external_value_set_exprt mod(
+        external_value_set_exprt evs_non_initializer(
           to_external_value_set(objexpr).as_non_initializer());
-        replacements.push_back({obj.first, mod});
+        replacements.push_back({obj.first, evs_non_initializer});
       }
     }