RFR: fix for alias analysis with ShenandoahBarriersForConst

[Roland Westrelin]

> *) I would prefer us to have large blocks of Shenandoah code in shenandoahSupport.?pp. For example,
> can we move AddPNode::shenandoah_fix_type to ShenandoahBarrierNode::fix_type? Then we can make the
> calls to it explicitly guarded with UseShenandoahGC:
>
>   Type* r = tp->add_offset(txoffset);
>   if (UseShenandoahGC) {
>     r = ShenandoahBarrierNode::fix_type(r);
>   }
>   return r;
>
> *) PhaseIdealLoop::place_near_use again: you mentioned this is a bugfix here
>   http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-February/004982.html
>
>  Do we want to split it out into a separate changeset? So we can e.g. backport it separately for
> sh/jdk10? Otherwise it keeps getting reverted along with other bulk changes, like it did in sh/jdk10
> when we reverted improved profile predicates...

Thanks for looking at this. I'll follow both suggestions.

Roland.