RFR: 8365024: G1: Make G1CollectionSet::_inc_build_state assert-only

[Thomas Schatzl]

Hi all,

  please review this change that makes the `G1CollectionSet::_inc_build_state` member debug/assert-only - it's only ever used in assertions.

Testing: local compilation, gha

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/26670

Commit messages:
 - 8365024

Changes: https://git.openjdk.org/jdk/pull/26671/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26671&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365024
  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26671.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26671/head:pull/26671

PR: https://git.openjdk.org/jdk/pull/26671