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

[Roman Kennke]

Looks ok. Thanks!
Roman

> 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
>