Test disjoint via jl_has_empty_intersection

This should be fast, as it has an `obviously_disjoint`-based fast path.

src/jltypes.c

@@ -718,6 +718,17 @@ jl_value_t *simple_union(jl_value_t *a, jl_value_t *b)
 }
 
 int obviously_disjoint(jl_value_t *a, jl_value_t *b, int specificity);
+
+static int simple_disjoint(jl_value_t *a, jl_value_t *b, int hasfree) {
+    if (jl_is_uniontype(b))
+        return simple_disjoint(a, ((jl_uniontype_t *)b)->a, hasfree) &&
+               simple_disjoint(a, ((jl_uniontype_t *)b)->b, hasfree);
+    else if (!hasfree && !jl_has_free_typevars(b))
+        return jl_has_empty_intersection(a, b);
+    else
+        return obviously_disjoint(a, b, 0);
+}
+
 jl_value_t *simple_intersect(jl_value_t *a, jl_value_t *b, int overesi)
 {
     // Unlike `Union`, we don't unwrap `UnionAll` here to avoid possible widening.
@@ -733,7 +744,7 @@ jl_value_t *simple_intersect(jl_value_t *a, jl_value_t *b, int overesi)
     size_t i, j;
     // first remove disjoint elements.
     for (i = 0; i < nt; i++) {
-        if (obviously_disjoint(temp[i], (i < nta ? b : a), 0))
+        if (simple_disjoint(temp[i], (i < nta ? b : a), jl_has_free_typevars(temp[i])))
             temp[i] = NULL;
     }
     // then check subtyping.

test/subtype.jl

@@ -2496,8 +2496,8 @@ end
 @test !<:(Tuple{Type{Union{Int, Val{1}}}, Int}, Tuple{Type{Union{Int, T1}}, T1} where T1<:Union{Val,Pair})
 @test <:(Tuple{Type{Union{Int, Val{1}}}, Int}, Tuple{Type{Union{Int, T1}}, T1} where T1<:Union{Integer,Val})
 @test <:(Tuple{Type{Union{Int, Int8}}, Int}, Tuple{Type{Union{Int, T1}}, T1} where T1<:Integer)
-@test_broken !<:(Tuple{Type{Union{Pair{Int, Any}, Pair{Int, Int}}}, Pair{Int, Any}},
-                 Tuple{Type{Union{Pair{Int, Any}, T1}}, T1} where T1<:(Pair{T,T} where {T}))
+@test !<:(Tuple{Type{Union{Pair{Int, Any}, Pair{Int, Int}}}, Pair{Int, Any}},
+          Tuple{Type{Union{Pair{Int, Any}, T1}}, T1} where T1<:(Pair{T,T} where {T}))
 
 let A = Tuple{Type{T}, T, Val{T}} where T,
     B = Tuple{Type{S}, Val{S}, Val{S}} where S