monitors_on_stack extracts unprocessed oops

While investigating JDK-8298058 we've found that monitors_on_stack extracts oops without setting up the RegisterMap to process oops. This then leaks out stale oops.

A simple fix is to change the RegisterMap to perform oops processing. However, monitors_on_stack is only used in an assert, so this means that we'll get a difference in behavior between release builds and debug builds. This has the potential to hide bugs in debug builds. It has been suggested to me that it might be better to simply remove the assert:
```
  assert(monitors_on_stack(current) == ((current->held_monitor_count() - current->jni_monitor_count()) > 0),
         "Held monitor count and locks on stack invariant: " INT64_FORMAT " JNI: " INT64_FORMAT, (int64_t)current->held_monitor_count(), (int64_t)current->jni_monitor_count());
```