G1: Merge multiple calls of get_next_marked_addr in HeapRegion::oops_on_memregion_iterate_in_unparsable

In `HeapRegion::oops_on_memregion_iterate_in_unparsable`:

```
  if (!bitmap->is_marked(cur)) {
    cur = bitmap->get_next_marked_addr(cur, end);
  }

  while (cur != end) {
    ...
    cur = bitmap->get_next_marked_addr(cur, end);
  }
  ...
```

Both this method and `get_next_marked_addr` are annotated with `inline`. Merge these two call sites into one to reduce #instruction in the assembly.