RFR: 8340186: Shenandoah: Missing load_reference_barrier_phantom_narrow match in is_shenandoah_lrb_call

[Aleksey Shipilev]

[JDK-8256999](https://bugs.openjdk.org/browse/JDK-8256999) added `ShenandoahRuntime::load_reference_barrier_phantom_narrow`, but missed adding it to `ShenandoahBarrierSetC2::is_shenandoah_lrb_call`. It is currently innocuous, as there are no users of `is_shenandoah_lrb_call`, but it will be important when [JDK-8340183](https://bugs.openjdk.org/browse/JDK-8340183) lands.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21016/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21016&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340186
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21016.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21016/head:pull/21016

PR: https://git.openjdk.org/jdk/pull/21016