The code inadvertently changed when parallel reference processing was
extended to all parallel collectors. Collectors such as ParNewGC and ParallelGC
which use in-place marking in the header do not need to set the
_is_alive_non_header field since the test therein becomes
redundant wrt checks already performed inline while scanning
Reference subtypes. Not doing the redundant test can marginally
speed up performance in the cases where the referent is
not reachable and in the young generation.

The performance impact as a result of this is usually in the noise
except for configurations where Reference subtypes are plentiful
and one uses very large Eden and/or Survivor Spaces.