RFR(XXS) 8220161: Shenandoah does not need to initialize PLABs for safepoint workers

[zgu at redhat.com]

Please review this small patch that not creating PLAB for safepoint
workers, it does not seem they evacuate anything.

It also fixes the crashes we have seen recently, because we never
enable PLAB creation for late safepoint workers, that results trying to
retire NULL PLAB during GC. 


Bug: https://bugs.openjdk.java.net/browse/JDK-8220161
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8220161/webrev.00/


Test:

  tier1, 2, 3_gc_shenandoah on Linux 64 (fastdebug and release)

Thanks,

-Zhengyu