optimization for subquery and function scan nodes: we can't just do it
unconditionally, we still have to check whether there is any need for
a whole-row Var.  I had been thinking that these node types couldn't
have any system columns, which is true, but that loop is also checking
for attno zero, ie, whole-row Var.  Fix comment to not be so misleading.
Per test case from Richard Huxton.