RFR: 8258382: Fix optimization-unstable code

[Hao Sun]

Optimization-unstable code refers to code that is unexpectedly discarded
by compiler optimizations due to undefined behavior in the program.

We applied a static checker called STACK (prototype from SOSP'13 paper
[1]) to OpenJDK source code and found the following eight sites of
potential unstable code.

Removing undefined behaviors would make the code stable.

[1] https://css.csail.mit.edu/stack/

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Note that we tested locally Jtreg tests ( tier1 and jdk::tier3) were passed on Linux x86/aarch64 machines after apply this patch.

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Commit messages:
 - 8258382: Fix optimization-unstable code

Changes: https://git.openjdk.java.net/jdk/pull/1886/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1886&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258382
  Stats: 16 lines in 5 files changed: 0 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1886.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1886/head:pull/1886

PR: https://git.openjdk.java.net/jdk/pull/1886