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DUALNUM: Improve/fix edge cases of unary minus.

Thanks to Sergey Kaplun.

(cherry picked from commit 707c12bf0)

In a single-number build (default for x86/x64), `-(0)` is always `-0`
due to its number type. However, in DUALNUM mode, the `0` may have the
integer type, and for this integer, `BC_UNM` has no effect. `0` may be
represented via number or integer type, where the resulting `0` is
constructed during initial parsing (`lj_strscan_scan()`) or arithmetic
operations (`BC_MODVV`). This inconsistency may break the JIT semantics
and lead to the assertion failure in the `lj_check_slots()`.

This patch fixes the incorrect `BC_UNM` for integer operand 0. Also, the
narrowing optimization for the unary minus should be more strict to
avoid incorrect narrowing for integer slots and make JIT intact with the
VM semantics.

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

Part of tarantool/tarantool#12134

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