RFR: 8254028: G1 incorrectly updates scan_top for collection set regions during preparation of evacuation

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes values of G1RemSetScanState::_scan_top for regions in the initial
  collection set consistent with ones in optional collection set?
So currently G1RemSetScanState::_scan_top is top() for regions in the initial collection set although they will never
be scanned as enforced when dropping the remsets onto the card table. For the optional collection set, G1 sets scan_top
manually to NULL when selecting the next few optional regions (using G1RemSet::exclude_region_from_scan()).

When debugging this discrepancy recently posed some slight surprise for me, so I would like to change this. Also there
is some risk that future code that relies on that property will be surprised.

Testing: tier1-4; lots of kitchensink runs with JDK-8254164.

Thanks,
  Thomas

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/557/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=557&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254028
  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/557.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/557/head:pull/557

PR: https://git.openjdk.java.net/jdk/pull/557