RFR (S): 8155233: Lazy coarse map clear

[Thomas Schatzl]

Hi all,

  can I have reviews for this tiny fix?

A significant part of remembered set clearing is spent on clearing the
coarse remembered set, particularly on large heaps.

One way to avoid this work most of the time is to only clear the coarse
bitmaps if they have actually been dirtied.

CR:
https://bugs.openjdk.java.net/browse/JDK-8155233
Webrev:
http://cr.openjdk.java.net/~tschatzl/8155233/webrev/
Testing:
jprt, vm.gc, perf tests

Thanks,
  Thomas