8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations

[rm-gh-8]

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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) and adjusted tests (passed). Patch is clean.

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Progress

• [x] Change must not contain extraneous whitespace
• [x] Commit message must refer to an issue
• [ ] JDK-8357822 needs maintainer approval

Issue

• JDK-8357822: C2: Multiple string optimization tests are no longer testing string concatenation optimizations (Bug - P4)

Reviewing

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$ git checkout pull/4130

Update a local copy of the PR:
$ git checkout pull/4130
$ git pull https://git.openjdk.org/jdk17u-dev.git pull/4130/head

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$ git pr checkout 4130

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