[subset] Compute set max using previous()

src/hb-subset-plan.cc

@@ -442,10 +442,9 @@ _create_old_gid_to_new_gid_map (const hb_face_t *face,
     | hb_sink (reverse_glyph_map)
     ;
 
-    unsigned max_glyph =
-    + hb_iter (all_gids_to_retain)
-    | hb_reduce (hb_max, 0u)
-    ;
+    hb_codepoint_t max_glyph = HB_SET_VALUE_INVALID;
+    hb_set_previous (all_gids_to_retain, &max_glyph);
+
     *num_glyphs = max_glyph + 1;
   }