RFR: 8068971: A heap region being cleared should not belong to the cset

[Erik Helin]

Hi all,

this tiny patch ensures that a heap region being cleared via
HeapRegion::hr_clear does not belong to the collection set. This has
never been the case, because we are only clearing heap regions upon
initialization (and then they don't belong to any cset) or from the
cleanup phase of the concurrent marking. The cleanup phase will only
call hr_clear on an old heap region, and an old heap region is only part
of the cset during a mixed GC (which can't happen concurrently with the
cleanup phase of concurrent marking).

Webrev:
http://cr.openjdk.java.net/~ehelin/8068971/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8068971

Testing:
- JPRT
- Running all jtreg tests locally (Linux x86-64)
- Running GCBasher locally
- Aurora:
  - Regression:
    - hotspot
    - nashorn
  - Bigapps:
    - DaCapo
    - Kitchensink
    - Weblogic
    - runThese
  - VM:
    - regression
    - stress
    - compiler
    - gc
    - oom
    - parallel class loading
    - quick
    - runtime

Thanks,
Erik