Fix improper repetition of previous results from a hashed aggregate.

ExecReScanAgg's check for whether it could re-use a previously calculated
hashtable neglected the possibility that the Agg node might reference
PARAM_EXEC Params that are not referenced by its input plan node.  That's
okay if the Params are in upper tlist or qual expressions; but if one
appears in aggregate input expressions, then the hashtable contents need
to be recomputed when the Param's value changes.

To avoid unnecessary performance degradation in the case of a Param that
isn't within an aggregate input, add logic to the planner to determine
which Params are within aggregate inputs.  This requires a new field in
struct Agg, but fortunately we never write plans to disk, so this isn't
an initdb-forcing change.

Per report from Jeevan Chalke.  This has been broken since forever,
so back-patch to all supported branches.

Andrew Gierth, with minor adjustments by me

Report: <CAM2+6=VY8ykfLT5Q8vb9B6EbeBk-NGuLbT6seaQ+Fq4zXvrDcA@mail.gmail.com>

src/backend/optimizer/plan/createplan.c

@@ -5664,6 +5664,7 @@ make_agg(List *tlist, List *qual,
 	node->grpColIdx = grpColIdx;
 	node->grpOperators = grpOperators;
 	node->numGroups = numGroups;
+	node->aggParams = NULL;		/* SS_finalize_plan() will fill this */
 	node->groupingSets = groupingSets;
 	node->chain = chain;