RFR: 8277556: Call ReferenceProcessorPhaseTimes::set_processing_is_mt once

[Albert Mingkun Yang]

On Mon, 22 Nov 2021 13:44:10 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change of moving the call site of `set_processing_is_mt`.
> 
> Test: hotspot_gc

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/6500