RFR: Pre-filter oops before enqueing them in SBS slowpaths

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/sbs-prefilter/webrev.01/

The comment says it all:

  // Filter marked objects before hitting the SATB queues. The same predicate would
  // be used by SATBMQ::filter to eliminate already marked objects downstream, but
  // filtering here helps to avoid wasteful SATB queueing work to begin with.
  if (!_heap->requires_marking(obj)) return;

This cuts down ref_array_pre lags around 3x on specjbb.

Testing: tier3_gc_shenandoah, eyeballing SBS lags on specjbb.

Thanks,
-Aleksey