Replace markWord.is_neutral() with markWord.is_unlocked()

In markword.hpp the definitions for is_neutral() and is_unlocked() are identical and always have been:

  bool is_unlocked() const {
    return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
  }
  bool is_neutral() const {
    return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
  }

Investigating this historical anomaly it seems in the early days of biased-locking, when different approaches were being trialed, the use of is_neutral slipped in via a merge and the anomaly was never noticed, or subsequently cleaned up. With a lot of work happening in this area of the code recently this anomaly has been causing some confusion, so it is prudent to clean things up and just replace is_neutral with is_unlocked where appropriate.