RFR: 8264818: G1: Improve liveness check for empty pinned regions after full gc marking [v3]

[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/openjdk/jdk/commit/47d740d244b09b104df635c07ee49f89f480ebc6).

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Fix merge error in comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3485/files
  - new: https://git.openjdk.java.net/jdk/pull/3485/files/edd03164..f7a44ddc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3485&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3485&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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