RFR: 8254314: Shenandoah: null checks in c2 should not skip over native load barrier

[Roland Westrelin]

C2 optimizes (CmpP (LoadBarrier o) NULL) as (CmpP o NULL). The ative
load barrier is not guaranteed to return a non null oop when passed a
non null oop so this optimization could lead to a crash.

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Commit messages:
 - Shenandoah: null checks in c2 should not skip over native load barrier

Changes: https://git.openjdk.java.net/jdk/pull/576/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=576&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254314
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/576.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/576/head:pull/576

PR: https://git.openjdk.java.net/jdk/pull/576