improve use of limit in stopping method search

We know that dispatch tuple signatures are always disjoint and
most-specific over their intersection, so they are always going to be
included in the result. We can use that knowledge to abort the search
quickly as we discover that there will be many specific matches.

src/gf.c

@@ -2507,6 +2507,7 @@ struct ml_matches_env {
     struct typemap_intersection_env match;
     int intersections;
     size_t world;
+    int lim;
     // results:
     jl_value_t *t; // array of svec(argtypes, params, Method, fully-covers)
     size_t min_valid;
@@ -2540,6 +2541,11 @@ static int ml_matches_visitor(jl_typemap_entry_t *ml, struct typemap_intersectio
             closure->max_valid = ml->max_world;
     }
     jl_method_t *meth = ml->func.method;
+    if (closure->lim >= 0 && jl_is_dispatch_tupletype(meth->sig)) {
+        if (closure->lim == 0)
+            return 0;
+        closure->lim--;
+    }
     closure->matc = jl_svec(4, closure->match.ti, closure->match.env, meth, closure->match.issubty ? jl_true : jl_false);
     size_t len = jl_array_len(closure->t);
     if (len == 0) {
@@ -2576,7 +2582,7 @@ static jl_value_t *ml_matches(jl_methtable_t *mt, int offs,
         else
             va = NULL;
     }
-    struct ml_matches_env env = {{ml_matches_visitor, (jl_value_t*)type, va}, intersections, world};
+    struct ml_matches_env env = {{ml_matches_visitor, (jl_value_t*)type, va}, intersections, world, lim};
     env.match.ti = NULL;
     env.match.env = jl_emptysvec;
     env.t = jl_an_empty_vec_any;
@@ -2639,7 +2645,10 @@ static jl_value_t *ml_matches(jl_methtable_t *mt, int offs,
             return env.t;
         }
     }
-    jl_typemap_intersection_visitor(defs, offs, &env.match);
+    if (!jl_typemap_intersection_visitor(defs, offs, &env.match)) {
+        JL_GC_POP();
+        return jl_false;
+    }
     *min_valid = env.min_valid;
     *max_valid = env.max_valid;
     // done with many of these values now