C2 verification pass shenandoah barriers

[Roman Kennke]

Hi Roland,


> > It's ok to commit if you change the read barriers in acmp to write
> > barriers.
> Here is a new webrev with the acmp barrier change and some other
> fixes
> because of a bug in the verification code:
> 
> http://cr.openjdk.java.net/~roland/shenandoah-verif/webrev.01/
> 
> Patch of the changes from the previous webrev is below.

Looks good!

Cheers,
Roman