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

[Daniel Skantz]

> This PR updates a few tests to reintroduce testing of string concatenation optimizations since a few bugs have recently been identified in this area.
> 
> Selection criteria: performed a text search on the test suite and identified tests for string concatenations or string optimizations that are not currently compiled with `-XDstringConcat=inline` and are not using StringBuilders explicitly.
> 
> Testing: T1-4.
> 
> Extra testing: ran the tests manually with `-XX:+PrintOptimizeStringConcat` and verified that the tests are exercising string optimizations after the fix.

Daniel Skantz has updated the pull request incrementally with two additional commits since the last revision:

 - revert change to TestStringIntrinsics.java
 - Update test/hotspot/jtreg/compiler/c2/Test7046096.java
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25610/files
  - new: https://git.openjdk.org/jdk/pull/25610/files/731667f4..7fd8568a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25610&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25610&range=00-01

  Stats: 13 lines in 2 files changed: 0 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25610.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25610/head:pull/25610

PR: https://git.openjdk.org/jdk/pull/25610