only infer concrete union-split signatures to cut down on inference

reduce O(n^2) behavior of `jl_recache_types`

these help #24383

base/reflection.jl

@@ -515,7 +515,10 @@ function _methods_by_ftype(t::ANY, lim::Int, world::UInt, min::Array{UInt,1}, ma
         end
     end
     if 1 < nu <= 64
-        return _methods_by_ftype(Any[tp...], t, length(tp), lim, [], world, min, max)
+        ms = _methods_by_ftype(Any[tp...], t, length(tp), lim, [], world, min, max)
+        if all(m->isleaftype(m[1]), ms)
+            return ms
+        end
     end
     # XXX: the following can return incorrect answers that the above branch would have corrected
     return ccall(:jl_matching_methods, Any, (Any, Cint, Cint, UInt, Ptr{UInt}, Ptr{UInt}), t, lim, 0, world, min, max)

src/dump.c

@@ -3035,6 +3035,8 @@ static jl_datatype_t *jl_recache_type(jl_datatype_t *dt, size_t start, jl_value_
         t = dt;
     }
     assert(t->uid != 0);
+    if (t == dt && v == NULL)
+        return t;
     // delete / replace any other usages of this type in the backref list
     // with the newly constructed object
     size_t i = start;