RFR: 8329764: G1: Handle null references during verification first [v2]

[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.

Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18650/files
  - new: https://git.openjdk.org/jdk/pull/18650/files/a6eecee0..a6eecee0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18650&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18650&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 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