RFR: 8261804: Remove field _processing_is_mt, calculate it instead [v2]

[Kim Barrett]

On Thu, 25 Feb 2021 14:35:08 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

>> In the reference processor, remove _processing_is_mt. Instead calculate its value in its accessor, processing_is_mt().
>> 
>> This change will remove some state, make RefProcMTDegreeAdjuster a bit simpler and make it much easier to derive when processing is indeed multi threaded.
>
> Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixup suggested by Kim

Marked as reviewed by kbarrett (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/2704