RFR: Trivial enhancement to avoid costly deletion array element

[Zhengyu Gu]

Trivial enhancement to avoid deletion array element, which may need 
costly shifting tailing elements.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/replace_nm/webrev.00/


Test:

   tier3_gc_shenandoah (fastdebug + release)

Thanks,

-Zhengyu