GIN jsonpath scans miss rows when methods follow a predicate

Hi,

Codex reported me an issue where GIN scans can miss matching rows when
methods are applied to a jsonpath predicate's result. For example:

------------------------------------------
CREATE TEMP TABLE t AS SELECT '2'::jsonb AS j;

-- Returns one row.
SELECT * FROM t WHERE j @@ '($ == 1).size().boolean()';

CREATE INDEX ON t USING gin (j);
SET enable_seqscan = off;

-- Returns no rows, but should return one.
SELECT * FROM t WHERE j @@ '($ == 1).size().boolean()';

RESET enable_seqscan;
------------------------------------------

In the default lax mode, "($ == 1).size().boolean()" evaluates to true.
But, extract_jsp_bool_expr() ignores the following methods and
incorrectly extracts an index condition from "$ == 1", causing the issue.

The attached patch fixes this by skipping index condition extraction for
predicates with following items.

Regards,

--
Fujii Masao