FTR: fix for TCK crash

[Roland Westrelin]

> So is_top() misses the some transformations that are captured by
> phase->type?

is_top() checks whether the node is the unique top node (which has type
TOP). When a node is found to have type TOP, it should be replaced with
the top node but there's a corner case here, where the node is not top
but has type TOP and is not replaced by the top node (I think because
the code being optimized is dead).

Roland.