RFR (S): Shortcut partial GC when no work expected

[Aleksey Shipilev]

Hi,

If we want to get partial as the default heuristics, then we better make sure it
does not do redundant work. For example, if we have no candidate regions, it
makes sense to shortcut the cycle:
  http://cr.openjdk.java.net/~shade/shenandoah/partial-shortcut/webrev.01/


Pause Partial GC (N)  =     0.07 s (a =     1503 us) (n =    45)
    (lvls, us =      904,     1152,     1426,     1719,     2331)

Pause Partial GC (N)  =     0.02 s (a =      517 us) (n =    43)
    (lvls, us =      193,      344,      459,      666,      894)

Thanks,
-Aleksey