ExecInitAgg: update aggstate->numaggs and ->numtrans earlier.

Functions hash_agg_entry_size() and build_hash_tables() make use of
those values for memory size estimates.

Because this change only affects memory estimates, don't backpatch.

Discussion: https://postgr.es/m/7530bd8783b1a78d53a3c70383e38d8da0a5ffe5.camel%40j-davis.com

src/backend/executor/nodeAgg.c

@@ -3379,8 +3379,8 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 		max_aggno = Max(max_aggno, aggref->aggno);
 		max_transno = Max(max_transno, aggref->aggtransno);
 	}
-	numaggs = max_aggno + 1;
+	aggstate->numaggs = numaggs = max_aggno + 1;
-	numtrans = max_transno + 1;
+	aggstate->numtrans = numtrans = max_transno + 1;
 
 	/*
 	 * For each phase, prepare grouping set data and fmgr lookup data for
@@ -3943,13 +3943,6 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 		ReleaseSysCache(aggTuple);
 	}
 
-	/*
-	 * Update aggstate->numaggs to be the number of unique aggregates found.
-	 * Also set numstates to the number of unique transition states found.
-	 */
-	aggstate->numaggs = numaggs;
-	aggstate->numtrans = numtrans;
-
 	/*
 	 * Last, check whether any more aggregates got added onto the node while
 	 * we processed the expressions for the aggregate arguments (including not