FTR: fix for TCK crash

[Aleksey Shipilev]

On 09/25/2017 02:30 PM, Roland Westrelin wrote:
>> 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).

Got it. Please make a useful commit message out of this explanation!

Thanks,
-Aleksey