RFR: 8297138: UB leading to crash in Amalloc with optimized builds

[Afshin Zafari]

8297138: UB leading to crash in Amalloc with optimized builds

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Commit messages:
 - debug_only() gaurd is removed.

Changes: https://git.openjdk.org/jdk/pull/11320/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11320&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297138
  Stats: 13 lines in 2 files changed: 6 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/11320.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11320/head:pull/11320

PR: https://git.openjdk.org/jdk/pull/11320