RFR: Reinstate reference processing shortcut, missing after merge

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/refproc-shortcut-reinstate/webrev.01/

This was accidentally removed during recent merges to sh/jdk. This shortcut is still effective,
because upstream work to mitigate these latencies had not yet landed in the mainline.

For example, Serial:

Without patch:

 Pause Final Mark (N) = 0.15 s (a = 1709 us) (n =    89)
   Finish Queues      = 0.01 s (a =  164 us) (n =    89)
   Weak References    = 0.12 s (a = 1386 us) (n =    89)
     Process          = 0.12 s (a = 1381 us) (n =    89)
   Prepare Evacuation = 0.01 s (a =  139 us) (n =    89)

With patch:

 Pause Final Mark (N) = 0.03 s (a =  334 us) (n =    95)
   Finish Queues      = 0.01 s (a =  103 us) (n =    95)
   Weak References    = 0.01 s (a =   83 us) (n =    95)  // <--- !!!
     Process          = 0.01 s (a =   79 us) (n =    95)
   Prepare Evacuation = 0.01 s (a =  130 us) (n =    95)

Testing: hotspot_gc_shenandoah, some benchmarks

Thanks,
-Aleksey