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Fix snapshot PC when linking to BC_JLOOP that was a BC_RET*.

Reported by Arseny Vakhrushev.
Fix contributed by Peter Cawley.

(cherry-picked from commit 5c46f4773)

As specified in the comment in `lj_record_stop`, all loops must
set `J->pc` to the next instruction. However, the chunk of logic
in `lj_trace_exit` expects it to be set to `BC_JLOOP` itself if
it used to be a `BC_RET`. This wrong pc results in the execution
of random data that goes after `BC_JLOOP` in the case of
restoration from the snapshot.

This patch fixes that behavior by adapting the loop recording
logic to this specific case.

NOTICE: This patch is only a part of the original commit,
and the other part is backported in the previous commit. The
patch was split into two, so the test case becomes easier to
implement since it can now depend on this assertion instead
of memory layout.

Maxim Kokryashkin:
* added the description and the test for the problem

Part of tarantool/tarantool#9145

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