Fix no-longer-valid shortcuts in expression_returns_set().

Fix no-longer-valid shortcuts in expression_returns_set().

expression_returns_set() used to short-circuit its recursion upon
seeing certain node types, such as DistinctExpr, that it knew the
executor did not support set-valued arguments for.  That was never
inherent, though, just a reflection of laziness in execQual.c.
With the new implementation of SRFs there is no reason to think
that any scalar-valued expression node could not have a set-valued
subexpression, except for AggRefs and WindowFuncs where we know there
is a parser check rejecting it.  And indeed, the shortcut causes
unexpected failures for cases such as a SRF underneath DistinctExpr,
because the planner stops looking for SRFs too soon.

Discussion: https://postgr.es/m/5259.1497044025@sss.pgh.pa.us