RFR: 8274264: Not all of G1 young collection verification honors the verification type

[Ahmed Muhsin]

Checking whether verification is required before running any of the verification steps.

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Commit messages:
 - check whether verification is required before running any verification steps

Changes: https://git.openjdk.org/jdk/pull/12818/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12818&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8274264
  Stats: 27 lines in 4 files changed: 12 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/12818.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12818/head:pull/12818

PR: https://git.openjdk.org/jdk/pull/12818