RFR: 8329764: G1: Handle null references during verification first

[Thomas Schatzl]

Hi all,

  please review this change that suggests to move the null reference check in object iteration during heap verification first.

The reason is as stated, since null references are fairly common (not only in that test mentioned in https://bugs.openjdk.org/browse/JDK-8329314), it may make sense to put it first. Also, null references never fail anyway (and verification failure is supposed to be uncommon).

Improves total runtime of that test case from 3s (~3.9s cpu time) to 2.2s (~3.1s cpu time).

If you think it makes the code too ugly, I will retract it.

Testing: gha, local testing.

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Depends on: https://git.openjdk.org/jdk/pull/18595

Commit messages:
 - 8329764

Changes: https://git.openjdk.org/jdk/pull/18650/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18650&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329764
  Stats: 12 lines in 1 file changed: 8 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18650.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18650/head:pull/18650

PR: https://git.openjdk.org/jdk/pull/18650