RFR: 8303084: G1 Heap region liveness verification has inverted return value

[Thomas Schatzl]

Hi all,

  please review a fix for a mess-up in [JDK-8303084](https://bugs.openjdk.org/browse/JDK-8303084) where the return value of `G1VerifyLiveAndRemSetClosure::verify_liveness()` is inverted.

Thanks to @albertnetymk for noticing.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/12721/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303084
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12721.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12721/head:pull/12721

PR: https://git.openjdk.org/jdk/pull/12721