RFR(S) 8219857: Shenandoah GC may initialize thread's gclab twice

[zgu at redhat.com]

Please review this small patch to avoid initializing early worker's
gclab twice.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219857
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8219857/webrev.00/

Test:
  tier1, 2, 3_gc_shenandoah on Linux x64 (fastdebug and release)

Thanks,

-Zhengyu