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

[Roman Kennke]

On Tue, 20 Oct 2020 14:07:28 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> 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

Looks good to me! Thank you!

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Marked as reviewed by rkennke (Reviewer).

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