RFR: 8255006: StringDedupTable lookup may encounter null oop with concurrent GC

[Zhengyu Gu]

Concurrent GC (e.g. Shenandoah) cleans string dedup table concurrently. It utilizes its load barrier to hide dead oop
(returns null instead), so that, StringDedupTable::loopup() may encounter null oops during table lookup.

- [x] tier1

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Commit messages:
 - 8255006: StringDedupTable lookup may encounter null oop with concurrent GC

Changes: https://git.openjdk.java.net/jdk/pull/743/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=743&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255006
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/743.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/743/head:pull/743

PR: https://git.openjdk.java.net/jdk/pull/743