RFR: 8255006: Add NULL-check to StringDedupTable lookup [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Update null check

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/743/files
  - new: https://git.openjdk.java.net/jdk/pull/743/files/fa749688..5d76d8bc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=743&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=743&range=00-01

  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