[jdk24u] RFR: 8347334: JimageDiffGenerator code clean-ups

[Severin Gehwolf]

Clean backport of some code clean-up for [JDK-8346239](https://bugs.openjdk.org/browse/JDK-8346239) which got recently backported to 24.0.1 as well.

Adds a test and improves code quality.

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Commit messages:
 - 8347334: JimageDiffGenerator code clean-ups

Changes: https://git.openjdk.org/jdk24u/pull/21/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk24u&pr=21&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347334
  Stats: 354 lines in 2 files changed: 327 ins; 14 del; 13 mod
  Patch: https://git.openjdk.org/jdk24u/pull/21.diff
  Fetch: git fetch https://git.openjdk.org/jdk24u.git pull/21/head:pull/21

PR: https://git.openjdk.org/jdk24u/pull/21