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FFI: Fix 64 bit shift fold rules.

Thanks to Peter Cawley.

(cherry picked from commit 9e0437240)

For `IR_BSHR`, `IR_BROL`, `IR_BROR` during `kfold_int64arith()` the left
argument is truncated down to 32 bits, which leads to incorrect results
if the right argument is >= 2^32.

Also, `IR_BSAR` does an unsigned shift rather than a signed shift, but
since this case branch is unreachable, it is harmless for now.

This patch fixes all misbehaviours (including possible for `IR_BSAR`) to
preserve IR semantics.

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

Part of tarantool/tarantool#10199

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