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Maintain chain invariant in DCE.

Thanks to Peter Cawley.

(cherry picked from commit f72c19e48)

Instructions with strong guards that are sometimes emitted with a guard
and sometimes emitted without a guard (like HREFK, CONV, or SLOAD) may
be eliminated from the IR chain and replaced with the NOP IR. If the
next IR of the same kind on the trace is not eliminated, it may
reference the IR NOP instead of an instruction of the same type. This
may lead to the corresponding assertion failure in the `rec_check_ir()`.

This patch unconditionally links the IRs during chain maintenance in
DCE.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9924

5661 of 6018 branches covered (94.07%)

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21610 of 23417 relevant lines covered (92.28%)

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