The second "if" block below was added in JDK-8267347

https://github.com/openjdk/jdk/blob/1864481df10d2f616cbfdecebf3bebbae04de5e1/src/hotspot/share/classfile/systemDictionaryShared.cpp#L1047

SystemDictionaryShared::record_linking_constraint() {
  ....

  if (!is_system_class_loader(klass_loader) &&
      !is_platform_class_loader(klass_loader)) {
     ...
    return;
  }

  if (DumpSharedSpaces && !is_builtin(klass)) {
    // During static dump, unregistered classes (those intended for
    // custom loaders) are loaded by the boot loader. Need to
    // exclude these for the same reason as above.
    // This should be fixed by JDK-8261941.
    return;
  }

Since JDK-8261941, unregistered classes are loaded by URLClassLoaders created by UnregisteredClasses::create_url_classloader(). Thus, they will be covered by the first "if" block.

So the second "if" block can be removed and replaced with

    assert(is_builtin(klass), "must be");