RFR: 8366117: Shenandoah: Notify barrier nodes when transitive inputs change during IGVN

[Kerem Kat]

`ShenandoahLoadReferenceBarrierNode::Identity()` determines if a barrier can be eliminated by traversing through intermediate nodes (`Phi`, `DecodeN`, `CastPP`, etc.) via `needs_barrier_impl()`. When these intermediate nodes change during IGVN, the barrier is not re-evaluated because IGVN only notifies direct users. This causes `TestVerifyIterativeGVN` to fail with Shenandoah, reporting missed optimization opportunities.

This fix adds a `BarrierSetC2::enqueue_dependent_gc_barriers()` virtual method that Shenandoah overrides to traverse outputs through the same node types and enqueue dependent barriers for re-evaluation.

There are more barrier eliminations enabled with this, e.g. in the arbitrarily chosen scrabble of renaissance benchmarks:


java -XX:+UseShenandoahGC -Xbatch -Xcomp -XX:-TieredCompilation -jar ~/.cache/stress/renaissance-gpl-0.16.1.jar scrabble -r 1 |& tee /dev/stderr | grep "optimize barrier" | sort | uniq -c | sort -rn

# with fix
236 optimize barrier on call
     75 optimize barrier on alloc
     35 optimize barrier on barrier
     23 optimize barrier on parm
     15 optimize barrier on const
      1 optimize barrier on barrier
      1 optimize barrier on null
TOTAL = 386

# without fix
193 optimize barrier on call
     72 optimize barrier on alloc
     32 optimize barrier on barrier
     23 optimize barrier on parm
      1 optimize barrier on barrier
      1 optimize barrier on null
      1 optimize barrier on const
TOTAL = 323


This requires uncommenting prints with `optimize barrier` in `c2/shenandoahSupport.cpp`.

Also tested with `make test CONF=linux-x86_64-server-fastdebug TEST=compiler/c2/TestVerifyIterativeGVN.java JTREG="VM_OPTIONS=-XX:+UseShenandoahGC -Xcomp -Xbatch -XX:-TieredCompilation"` which passes with the fix and fails without.

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Commit messages:
 - 8366117: Shenandoah: Notify barrier nodes when transitive inputs change during IGVN

Changes: https://git.openjdk.org/jdk/pull/29491/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366117
  Stats: 37 lines in 4 files changed: 37 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29491.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29491/head:pull/29491

PR: https://git.openjdk.org/jdk/pull/29491