RFR: 8341696: C2: Non-fluid StringBuilder pattern bails out in OptoStringConcat [v6]

[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 five additional commits since the last revision:

 - Update test/micro/org/openjdk/bench/vm/compiler/FluidSBBench.java
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
 - Update test/hotspot/jtreg/compiler/stringopts/TestFluidAndNonFluid.java
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
 - Update src/hotspot/share/opto/stringopts.hpp
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
 - Update src/hotspot/share/opto/stringopts.cpp
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
 - Update src/hotspot/share/opto/stringopts.cpp
   
   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22537/files
  - new: https://git.openjdk.org/jdk/pull/22537/files/c82970fb..50a66954

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22537&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22537&range=04-05

  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 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