Replace run-time error check with assertion

The error message was checking that the structures returned from the
parser matched expectations.  That's something we usually use
assertions for, not a full user-facing error message.  So replace that
with an assertion (hidden inside lfirst_node()).
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/452e9df8-ec89-e01b-b64a-8cc6ce830458%40enterprisedb.com

src/backend/commands/statscmds.c

@@ -220,26 +220,14 @@ CreateStatistics(CreateStatsStmt *stmt)
 	 */
 	foreach(cell, stmt->exprs)
 	{
-		Node	   *expr = (Node *) lfirst(cell);
-		StatsElem  *selem;
-		HeapTuple	atttuple;
-		Form_pg_attribute attForm;
-		TypeCacheEntry *type;
-
-		/*
-		 * We should not get anything else than StatsElem, given the grammar.
-		 * But let's keep it as a safety.
-		 */
-		if (!IsA(expr, StatsElem))
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("only simple column references and expressions are allowed in CREATE STATISTICS")));
-
-		selem = (StatsElem *) expr;
+		StatsElem  *selem = lfirst_node(StatsElem, cell);
 
 		if (selem->name)		/* column reference */
 		{
 			char	   *attname;
+			HeapTuple	atttuple;
+			Form_pg_attribute attForm;
+			TypeCacheEntry *type;
 
 			attname = selem->name;
 
@@ -273,6 +261,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 		{
 			Node	   *expr = selem->expr;
 			Oid			atttype;
+			TypeCacheEntry *type;
 
 			Assert(expr != NULL);