[jdk16u] RFR: 8258946: Fix optimization-unstable code involving signed integer overflow

[Tobias Hartmann]

Backport of [JDK-8258946](https://bugs.openjdk.java.net/browse/JDK-8258946). Applies cleanly.

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Commit messages:
 - 8258946: Fix optimization-unstable code involving signed integer overflow

Changes: https://git.openjdk.java.net/jdk16u/pull/9/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=9&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258946
  Stats: 10 lines in 3 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/9.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/9/head:pull/9

PR: https://git.openjdk.java.net/jdk16u/pull/9