Fix issues around EXPLAIN with JIT.

Fix issues around EXPLAIN with JIT.

I (Andres) was more than a bit hasty in committing 33001fd7a7072d48327
after last minute changes, leading to a number of problems (jit output
was only shown for JIT in parallel workers, and just EXPLAIN without
ANALYZE didn't work).  Lukas luckily found these issues quickly.

Instead of combining instrumentation in in standard_ExecutorEnd(), do
so on demand in the new ExplainPrintJITSummary().

Also update a documentation example of the JIT output, changed in
52050ad8ebec8d831.

Author: Lukas Fittl, with minor changes by me
Discussion: https://postgr.es/m/CAP53PkxmgJht69pabxBXJBM+0oc6kf3KHMborLP7H2ouJ0CCtQ@mail.gmail.com
Backpatch: 11, where JIT compilation was introduced