short circuit suggestion in find_hash_columns()

I was looking at find_hash_columns() in nodeAgg.c

It seems the first loop tries to determine the max column number needed, along with whether all columns are needed.

The loop can be re-written as shown in the patch.

In normal cases, we don't need to perform scanDesc->natts iterations.

In best case scenario, the loop would terminate after two iterations.

Please provide your comment.

Thanks