When multiplying 36 bits by 36 bits using four 18x18 multipliers, the
sign bits of the higher 18-bit parts of the multipliers were correctly
switched, but what was incorrect was leaving the sign bits of the lower
parts of the multipliers uninitialized. They now connect to VSS.
Addresses https://github.com/YosysHQ/apicula/issues/242
Signed-off-by: YRabbit <rabbit@yrabbit.cyou>
4d5c48ad83