[jdk25u] RFR: 8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations

[Francesco Andreuzzi]

Clean backport, adds several tests for string concatenation optimization in C2.

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Commit messages:
 - Backport 6c616c71ec9a8ee6e0203921deef20d09db39698

Changes: https://git.openjdk.org/jdk25u/pull/73/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=73&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/jdk25u/pull/73.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/73/head:pull/73

PR: https://git.openjdk.org/jdk25u/pull/73