[jdk21u] RFR: 8316428: G1: Nmethod count statistics only count last code root set iterated

[Jiangli Zhou]

Auto suggested description:
This pull request contains a backport of commit [fab372d3](https://github.com/openjdk/jdk/commit/fab372d3a23b17f64ae4306e28bdb0bc511f4912) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

The commit being backported was authored by Thomas Schatzl on 19 Sep 2023 and was reviewed by Ivan Walulya and Albert Mingkun Yang.

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Commit messages:
 - Backport fab372d3a23b17f64ae4306e28bdb0bc511f4912

Changes: https://git.openjdk.org/jdk21u/pull/458/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=458&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316428
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u/pull/458.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/458/head:pull/458

PR: https://git.openjdk.org/jdk21u/pull/458