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x86/x64: Don't use undefined MUL/IMUL zero flag.

Reported by VrIgHtEr.

(cherry picked from commit c92d0cb19)

When emitting the arithmetic operations on registers via the
`asm_intarith()`, the next `test` instruction may be dropped since the
flag register is modified by the arithmetic instruction to be emitted.

But the `imul` instruction [1] doesn't modify ZF, so its value is
undefined. This patch prevents dropping the `test` instruction if
the emitted instruction is `imul`.

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

[1]: https://www.felixcloutier.com/x86/imul

Part of tarantool/tarantool#11691

Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Signed-off-by: Sergey Kaplun <skaplun@tarantool.org>

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