BUG #17437: time spent in JIT is also attributed to function scan

The following bug has been logged on the website:

Bug reference:      17437
Logged by:          Jeff Janes
Email address:      jeff.janes@gmail.com
PostgreSQL version: 14.2
Operating system:   Linux
Description:

While looking into a stack overflow question:

https://stackoverflow.com/questions/71456871/new-aggregation-code-performs-worse-despite-using-1000x-less-data-with-postgres/71458545#71458545

I found a bug where time spent in setting up JIT is also attributed to
something else (the function scan over generate_series)

I can replicate the error (although not quite as dramatically--the problem
is obvious, but only a small fraction of the overall time unlike in the
original problem) using pgbench -i -s20 schema.

explain analyze select aid from generate_series('2022-02-25
14:00:00'::timestamp, '2022-02-25 15:00:00'::timestamp, interval '5min')
f(t) join pgbench_accounts on extract(epoch from (now()-t))*1000 < aid;


   QUERY PLAN
                 

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.43..67869958.43 rows=666666667 width=4) (actual
time=16825.375..16825.378 rows=0 loops=1)
   Join Filter: ((date_part('epoch'::text, (now() - (f.t)::timestamp with
time zone)) * '1000'::double precision) < (pgbench_accounts.aid)::double
precision)
   Rows Removed by Join Filter: 26000000
   ->  Function Scan on generate_series f  (cost=0.00..10.00 rows=1000
width=8) (actual time=1060.859..1060.881 rows=13 loops=1)
   ->  Materialize  (cost=0.43..69761.43 rows=2000000 width=4) (actual
time=0.009..395.611 rows=2000000 loops=13)
         ->  Index Only Scan using pgbench_accounts_pkey on pgbench_accounts
 (cost=0.43..51948.43 rows=2000000 width=4) (actual time=0.043..447.145
rows=2000000 loops=1)
               Heap Fetches: 0
 Planning Time: 0.157 ms
 JIT:
   Functions: 8
   Options: Inlining true, Optimization true, Expressions true, Deforming
true
   Timing: Generation 7.150 ms, Inlining 257.488 ms, Optimization 439.004
ms, Emission 352.708 ms, Total 1056.350 ms
 Execution Time: 17662.690 ms
(13 rows)

That it takes a second for generate_series to generate 13 rows is of course
ridiculous, and the time is suspiciously similar to the reported JIT time.
If I turn off jit, then the time reported in generate_series drops to
negligible.  This example is from v12, but I also replicated in v14 and
v15dev.  The exact timings differ, but the time attributed to the
generate_series is always very close to that attributed to JIT.