[jdk25u] RFR: 8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations [v2]

[Francesco Andreuzzi]

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

Francesco Andreuzzi has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Backport 6c616c71ec9a8ee6e0203921deef20d09db39698

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Changes:
  - all: https://git.openjdk.org/jdk25u/pull/73/files
  - new: https://git.openjdk.org/jdk25u/pull/73/files/b58ba062..92ea04d3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk25u&pr=73&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk25u&pr=73&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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