RFR: 7923: Cleanup warnings in core

[Marcus Hirt]

Cleanup to reduce the warning noise a bit.

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Commit messages:
 - 7923: Cleanup warnings in core

Changes: https://git.openjdk.org/jmc/pull/438/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=438&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7923
  Stats: 46 lines in 26 files changed: 28 ins; 17 del; 1 mod
  Patch: https://git.openjdk.org/jmc/pull/438.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/438/head:pull/438

PR: https://git.openjdk.org/jmc/pull/438