RFR: Silence compilation warnings on implicit type conversion

[Roman Kennke]

> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/silence_win_warnings/webrev.00/
> 
> Test:
>   Built on Windows.
> 
> Thanks,
> 
> -Zhengyu

Sure, ok. Thanks!
Roman