G1: Refactor G1BlockOffsetTablePart::block_at_or_preceding

```
  HeapWord* q = _bot->address_for_index(index); <--- init of q

  uint offset = _bot->offset_array(index); // Extend u_char to uint.
  while (offset >= BOTConstants::N_words) {
    // The excess of the offset from N_words indicates a power of Base
    // to go back by.
    size_t n_cards_back = BOTConstants::entry_to_cards_back(offset);
    q -= (BOTConstants::N_words * n_cards_back); <--- keep q in sync
    index -= n_cards_back;
    offset = _bot->offset_array(index);
  }
```

`G1BlockOffsetTablePart::block_at_or_preceding` identifies the value (`offset`) and the address (`q`) of the "offset card". The final result is just `q - offset`. Because the "block start" can be many cards before, it iterates backwards on the cards array to locate the "offset card". Along the way, `q` mirrors the current card. However, there's 1:1 mapping btw `q` and the current card. Therefore, one can derive `q` once the "offset card" is identified, instead of calculating `q` in the loop.