RFR (XS): 8140689: Skip last young-only gc if nothing to do in the mixed gc phase

[Thomas Schatzl]

Hi all,

  can I have reviews for another change that minimizes the turnaround
time between one marking round and another one?

In this case, if GC cleanup detects that there is nothing to reclaim, it
will immediately request an initial mark for the next young-only gc.
Previously there had been the requirement that the next young-only gc
after marking must be a "regular" young-only gc.
This wastes some time until we have a new up-to-date view of the
liveness, potentially ending in full gcs.

CR:
https://bugs.openjdk.java.net/browse/JDK-8140689
Webrev:
http://cr.openjdk.java.net/~tschatzl/8140689/webrev
Testing:
jprt, vm.gc, manual testing

Thanks,
  Thomas