Fix negative zero from I256 sdiv/srem with mixed-sign operands - #302
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Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
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When sdiv/srem operands have different signs and the result is zero, overflowing_div/overflowing_rem built
I256 { is_negative: true, abs: 0 }, and hashing that value overflows into_u256(!0 + 1), which panics SCCP when it folds e.g.evm_sdiv(-1, 5). Fixes #300 by canonicalizing zero inmake_negative.