Issue with point_ops and NaN

Hi,

While running some sanity checks on the regression tests, I found one test that
returns different results depending on whether an index or a sequential scan is
used.

Minimal reproducer:

=# CREATE TABLE point_tbl AS select '(nan,nan)'::point f1;
=# CREATE INDEX ON point_tbl USING gist(f1);

=# EXPLAIN SELECT * FROM point_tbl WHERE f1 <@ polygon '(0,0),(0,100),(100,100),(50,50),(100,0),(0,0)';
QUERY PLAN
------------------------------------------------------------------------------
Seq Scan on point_tbl (cost=0.00..1.01 rows=1 width=16)
Filter: (f1 <@ '((0,0),(0,100),(100,100),(50,50),(100,0),(0,0))'::polygon)
(2 rows)

=# SELECT * FROM point_tbl WHERE f1 <@ polygon '(0,0),(0,100),(100,100),(50,50),(100,0),(0,0)';
f1
-----------
(NaN,NaN)
(1 row)

SET enable_seqscan = 0;

=# EXPLAIN SELECT * FROM point_tbl WHERE f1 <@ polygon '(0,0),(0,100),(100,100),(50,50),(100,0),(0,0)';
QUERY PLAN
----------------------------------------------------------------------------------------
Index Only Scan using point_tbl_f1_idx on point_tbl (cost=0.12..8.14 rows=1 width=16)
Index Cond: (f1 <@ '((0,0),(0,100),(100,100),(50,50),(100,0),(0,0))'::polygon)
(2 rows)

=# SELECT * FROM point_tbl WHERE f1 <@ polygon '(0,0),(0,100),(100,100),(50,50),(100,0),(0,0)';
f1
----
(0 rows)

The discrepancy comes from the fact that the sequential scan checks the
condition using point_inside() / lseg_crossing(), while the gist index will
check the condition using box_overlap() / box_ov(), which have different
opinions on how to handle NaN.

Getting a consistent behavior shouldn't be hard, but I'm unsure which behavior
is actually correct.