A minor simplification for List manipulation

Fix one place that was using lfirst(list_head(list)) by using linitial(list)
instead.  They are equivalent but the latter is simpler.  We did the same in
9d299a49.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAMbWs49dJnpezDQDDxCPKq7+=_3NyqLqGqnhqCjd+dYe4MS15w@mail.gmail.com

src/backend/rewrite/rewriteSearchCycle.c

@@ -523,7 +523,7 @@ rewriteSearchAndCycle(CommonTableExpr *cte)
 
 			fexpr = makeFuncExpr(F_INT8INC, INT8OID, list_make1(fs), InvalidOid, InvalidOid, COERCE_EXPLICIT_CALL);
 
-			lfirst(list_head(search_col_rowexpr->args)) = fexpr;
+			linitial(search_col_rowexpr->args) = fexpr;
 
 			texpr = (Expr *) search_col_rowexpr;
 		}