Reactivate query flattening when the result set of the outer query has

Reactivate query flattening when the result set of the outer query has
no function calls or subqueries.  This is a partial reversal of
check-in [c9104b59].  Co-routines are still preferred if the outer
query has a complex result set, but for simple results sets, query flattening
is used.  Check-in [4464f40ccd7] is completely backed
out due to this change.

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