RFR: 8269294: Verify_before/after_young_collection should execute all verification

[Thomas Schatzl]

Hi all,

  can I have reviews for this tiny change that moves all verification from `do_collection_pause_at_safepoint_helper()` to `verify_before/after_young_collection`?

I actually removed one call to `_hrm.verify_optional()` because it is already called in `_verifier->verify_region_sets_optional` although they are not completely equivalent as one call is `DEBUG_ONLY` and the other `NOT_PRODUCT`, but nobody is probably using the optimized build anyway. I can re-add the missing call if required.

Testing: tier1

Thanks,
  Thomas

P.S: this is based on PR#4539, so it would be nice if it could be reviewed before this.

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Depends on: https://git.openjdk.java.net/jdk/pull/4539

Commit messages:
 - Fix young gc verify stages

Changes: https://git.openjdk.java.net/jdk/pull/4586/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4586&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269294
  Stats: 9 lines in 1 file changed: 3 ins; 6 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4586.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4586/head:pull/4586

PR: https://git.openjdk.java.net/jdk/pull/4586