In `HeapRegion::oops_on_memregion_seq_iterate_careful`:
```
  HeapWord* cur = block_start(start);
  ...
  {
    assert(cur <= start,
           "cur: " PTR_FORMAT ", start: " PTR_FORMAT, p2i(cur), p2i(start));
    HeapWord* next = cur + block_size(cur);
    assert(start < next,
           "start: " PTR_FORMAT ", next: " PTR_FORMAT, p2i(start), p2i(next));
  }
```
Those assertions are checking postconditions of `block_start` (after JDK-8283332).

However, the same checkings are already done inside `block_start(...)`.

```
  assert(q <= addr, "wrong order for q and addr");
  assert(addr < n, "wrong order for addr and n");
```

Remove the duplicate assertions in the caller.