Combining array slicing and indexing causes incorrect/confusing results

Due to the way SubscriptingRef works there's no way for it to describe
a subscript sequence that both does a slice and a regular index in the
same SubscriptingRef. This results into weird behaviour like the
following:

> select ('{1, 2, 3, 4}'::int[])[1:4][2];
 int4
──────
 {}
(1 row)

It's possible to get the expected result by adding some additional parenthesis

> select (('{1, 2, 3, 4}'::int[])[1:4])[2];
 int4
──────
    2
(1 row)