[lworld] RFR: 8256481: [lworld] C2 fails to eliminate GC barriers when replacing in line type buffer allocation

[Tobias Hartmann]

In Valhalla code, `G1BarrierSetC2::eliminate_gc_barrier` is not only used at macro expansion but also during IGVN to remove inline type buffer allocations. Current code is not robust enough to handle duplicate barriers and other IR shapes that were not yet folded by IGVN. This fix makes it more robust.

Thanks,
Tobias

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Commit messages:
 - 8256481: [lworld] C2 fails to eliminate GC barriers when replacing inline type buffer allocation

Changes: https://git.openjdk.java.net/valhalla/pull/269/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=269&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256481
  Stats: 36 lines in 2 files changed: 6 ins; 4 del; 26 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/269.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/269/head:pull/269

PR: https://git.openjdk.java.net/valhalla/pull/269