Use narrow instead of numeric_cast on std::size_t

numeric_cast implicitely converts the std::size_t to an mp_integer,
before converting it back to an integral value and checking for bound
which is less efficient than narrow.

Author: romainbrenguier

src/solvers/prop/bdd_expr.cpp

@@ -112,7 +112,7 @@ exprt bdd_exprt::as_expr(
       return true_exprt();
   }
 
-  auto index = numeric_cast_v<std::size_t>(r.index());
+  auto index = narrow<std::size_t>(r.index());
   INVARIANT(index < node_map.size(), "Index should be in node_map");
   const exprt &n_expr = node_map[index];