Tidy up code in get_cheapest_group_keys_order()

There are a few things that we could do a little better within
get_cheapest_group_keys_order():

1. We should be using list_free() rather than pfree() on a List.

2. We should use for_each_from() instead of manually coding a for loop to
skip the first n elements of a List

3. list_truncate(list_copy(...), n) is not a great way to copy the first n
elements of a list. Let's invent list_copy_head() for that.  That way we
don't need to copy the entire list just to truncate it directly
afterwards.

4. We can simplify finding the cheapest cost by setting the cheapest cost
variable to DBL_MAX.  That allows us to skip special-casing the initial
iteration of the loop.

Author: David Rowley
Discussion: https://postgr.es/m/CAApHDvrGyL3ft8waEkncG9y5HDMu5TFFJB1paoTC8zi9YK97Nw@mail.gmail.com
Backpatch-through: 15, where get_cheapest_group_keys_order was added.

src/backend/nodes/list.c

@@ -1584,6 +1584,27 @@ list_copy(const List *oldlist)
 	return newlist;
 }
 
+/*
+ * Return a shallow copy of the specified list containing only the first 'len'
+ * elements.  If oldlist is shorter than 'len' then we copy the entire list.
+ */
+List *
+list_copy_head(const List *oldlist, int len)
+{
+	List	   *newlist;
+
+	len = Min(oldlist->length, len);
+
+	if (len <= 0)
+		return NIL;
+
+	newlist = new_list(oldlist->type, len);
+	memcpy(newlist->elements, oldlist->elements, len * sizeof(ListCell));
+
+	check_list_invariants(newlist);
+	return newlist;
+}
+
 /*
  * Return a shallow copy of the specified list, without the first N elements.
  */