RFR: 8252589: Code duplication in ParallelSPCleanupTask

[Stefan Karlsson]

Hi all,

Please review this small patch to remove some code duplication in 
ParallelSPCleanupTask.

https://cr.openjdk.java.net/~stefank/8252589/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8252589

I noticed this while reviewing the patches for JEP 376. I think this 
makes it more apparent what the individual sub-tasks are doing.

Thanks,
StefanK