RFR: 8286297: G1: Simplify parallel and serial verification code paths

[Thomas Schatzl]

Hi all,

  please review this small change that simplifies the path splitting parallel/serial execution of verification.

Testing: gha, local testing

Thanks,
  Thomas

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Commit messages:
 - more code simplification
 - initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/8572/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8572&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286297
  Stats: 13 lines in 1 file changed: 0 ins; 8 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8572.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8572/head:pull/8572

PR: https://git.openjdk.java.net/jdk/pull/8572