RFR: 8340183: Shenandoah: LRB node is not matched as GC barrier after JDK-8340183

[Aleksey Shipilev]

[JDK-8340183](https://bugs.openjdk.org/browse/JDK-8340183) introduced a regression: `ShenandoahBarrierSetC2::is_gc_barrier_node` is now answering `false` for the actual `ShenandoahLoadReferenceBarrierNode`. The fix reinstates the check for LRB node.

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
 - [x] Linux x86_64 server fastdebug, `jdk/jfr/api/consumer/ ` (100x) -- used to fail intermittently, now it does not

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21266/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21266&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340183
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21266.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21266/head:pull/21266

PR: https://git.openjdk.org/jdk/pull/21266