[JDK11] RFR: Silence compilation warnings on implicit type conversion

[Zhengyu Gu]

This is Shenandoah JDK11 specific one, others should be fixed vs. 
Shenandoah JDK backport.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/silence_win_warnings_jdk11/webrev.00/

Test:

   Built on Windows.

Thanks,

-Zhengyu