RFR: 8277652: SIGSEGV in ShenandoahBarrierC2Support::verify_raw_mem for malformed control flow graph

[Jie Fu]

Hi all,

`ShenandoahBarrierC2Support::verify_raw_mem` crashes due to `u->unique_ctrl_out()` [1] returns NULL for malformed control flow graph.
It can be reproduced by running `compiler/vectorapi/TestIntrinsicBailOut.java` with `-XX:+UseShenandoahGC`.
It would be better to fix it.

Thanks.
Best regards,
Jie

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp#L1925

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Commit messages:
 - 8277652: SIGSEGV in ShenandoahBarrierC2Support::verify_raw_mem for malformed control flow graph

Changes: https://git.openjdk.java.net/jdk/pull/6525/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6525&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277652
  Stats: 13 lines in 2 files changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6525.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6525/head:pull/6525

PR: https://git.openjdk.java.net/jdk/pull/6525