inference: avoid world age assertion

This was over-aggressive. It was only correct in the case where we where
depending on the `.inferred` field or `.rettype` contents exactly.
However, sometimes inference of the method instead depends on `.def.source`.

fix #23981
fix #23768

Author: vtjnash

src/gf.c

@@ -1348,7 +1348,7 @@ static int invalidate_backedges(jl_typemap_entry_t *oldentry, struct typemap_int
 // add a backedge from callee to caller
 JL_DLLEXPORT void jl_method_instance_add_backedge(jl_method_instance_t *callee, jl_method_instance_t *caller)
 {
-    assert(callee->min_world <= caller->min_world && callee->max_world >= caller->max_world);
+    assert(callee->def.method->min_world <= caller->min_world && callee->max_world >= caller->max_world);
     JL_LOCK(&callee->def.method->writelock);
     if (!callee->backedges) {
         // lazy-init the backedges array