[jdk17u-dev] RFR: 8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations [v2]

[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.

Roland Mesde has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8357822-no-longer-testing-string-concatenation
 - Backport 6c616c71ec9a8ee6e0203921deef20d09db39698

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Changes:
  - all: https://git.openjdk.org/jdk17u-dev/pull/4130/files
  - new: https://git.openjdk.org/jdk17u-dev/pull/4130/files/8935e9f1..afcf43c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4130&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4130&range=00-01

  Stats: 8166 lines in 139 files changed: 5115 ins; 2633 del; 418 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