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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.

As specified in lj_record.c:304, all loops must set `J->pc` to
the next instruction. However, the chunk of logic at
lj_trace.c:923 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.

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

Part of tarantool/tarantool#8825

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