ibc/binary: Fix the calculation of "compressed"

We determine whether or not an instruction is compressed based on how
many registers it writes.  We compute this value based on the byte mask
of bytes written by the instruction.  In particular, whether we write to
just one register or tso.  Unfortunately, we computed this byte mask by
left-shifting a 32-bit constant so we never saw any writes to the second
register. 🤦
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