[jdk17u-dev] RFR: 8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations

[Roland Mesde]

Backporting JDK-8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations. Adds several tests for string concatenation optimization in C2. Ran GHA Sanity Checks (passed except for one macOS specific check) and adjusted tests (passed). Sanity check failure per-existing (affects other unrelated PRs created prior to this PR), and unrelated to this PR which adds tests. Patch is clean.

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Commit messages:
 - Backport 6c616c71ec9a8ee6e0203921deef20d09db39698

Changes: https://git.openjdk.org/jdk17u-dev/pull/4130/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357822
  Stats: 86 lines in 6 files changed: 81 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4130.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4130/head:pull/4130

PR: https://git.openjdk.org/jdk17u-dev/pull/4130