RFR: 8341696: C2: Non-fluid StringBuilder pattern bails out in OptoStringConcat [v9]

[Theo Weidmann]

> Extends stringopts to also recognize non-fluid uses of StringBuilder and optimize them the same way.
> 
> For example, this basic case was not optimized before and is optimized with this PR:
> 
> 
> StringBuilder sb = new StringBuilder();
> sb.append("a");
> sb.append(a);
> return sb.toString();

Theo Weidmann has updated the pull request incrementally with one additional commit since the last revision:

  Fix merge gone wrong

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22537/files
  - new: https://git.openjdk.org/jdk/pull/22537/files/a2c3b727..ea04c9dc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22537&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22537&range=07-08

  Stats: 141 lines in 1 file changed: 78 ins; 43 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/22537.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22537/head:pull/22537

PR: https://git.openjdk.org/jdk/pull/22537