RFR: Matrix should assert indexes to be in bounds

[Zhengyu Gu]

Looks good.

-Zhengyu

On 06/27/2017 02:16 PM, Aleksey Shipilev wrote:
> This serves as protection against matrix overflows:
>   http://cr.openjdk.java.net/~shade/shenandoah/matrix-verify-index/webrev.01/
>
> Testing: hotspot_gc_shenandoah
>
> Thanks,
> -Aleksey
>