RFR: 8264818: G1: Improve liveness check for empty pinned regions after full gc marking

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change improving the check for empty Open Archive regions using information that has recently been added to the full gc in [JDK-8263495](https://bugs.openjdk.java.net/browse/JDK-8263495) instead of scanning the region?

Testing: tier1-3, manual check that the freeing still works using some hack to create a dummy Open Archive region which contents are completely unreferenced and then running a "Hello World" app doing a system.gc. The patch for the VM to create that archive is [here](https://github.com/tschatzl/jdk/tree/a8264818-create-dummy-open-archive).

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/3485/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3485&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264818
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3485.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3485/head:pull/3485

PR: https://git.openjdk.java.net/jdk/pull/3485