RFR: 8300968: Accessorize raw oop load in DeadCounterClosure

[Erik Österlund]

The DeadCounterClosure class uses raw loads to check for dead phantom oop* entries. This isn't completely GC independent (because generational ZGC colours null pointers as well). We should accessorize this access so that all GCs become happy.

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Commit messages:
 - 8300968: Accessorize raw oop load in DeadCounterClosure

Changes: https://git.openjdk.org/jdk/pull/12168/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300968
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12168.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12168/head:pull/12168

PR: https://git.openjdk.org/jdk/pull/12168