minor performance tuning;

--- a/src/Pure/Isar/generic_target.ML	Mon Jan 08 21:46:26 2024 +0100
+++ b/src/Pure/Isar/generic_target.ML	Mon Jan 08 21:46:43 2024 +0100
@@ -342,7 +342,7 @@
           (case v of
             Var (xi, T) =>
               Vars.add ((xi, Term_Subst.instantiateT instT T),
-                Thm.cterm_of lthy' (Term.map_types (Term_Subst.instantiateT instT) t))
+                Thm.cterm_of lthy' (Term.map_types (Term_Subst.instantiateT_same instT) t))
           | _ => I)) vars frees);
     val fixed_thm = Thm.instantiate (cinstT, cinst) global_thm;