[backport] 8237007: Shenandoah: assert(_base == Tuple) failure during C2 compilation

[Roman Kennke]

Looks good to me.

Roman

> It's been integrated in jdk/jdk 2 days ago and hasn't caused any follow
> up bug.
> 
> http://cr.openjdk.java.net/~roland/8237007-shenandoah-11u/webrev.00/
> 
> Roland.
>