[11] RFR(XS): 8202179: Compilation fails with assert(n->is_expensive()) failed: expensive nodes with non-null control here only

[Roland Westrelin]

> The reported problem is in the exceptional case when the SqrtDNode is found to be unique (there is
> no copy with the same inputs) and therefore the control input is removed. If we only then create a
> SqrtFNode in Ideal(), we would use that NULL control as input which violates the assumption that
> expensive nodes always have a non-NULL control.

I get it now. That looks good to me.

Roland.