ice40: Fix carry feed-out when we have to split the chain next
ice40: Fix carry feed-out when we have to split the chain next

Signed-off-by: David Shah <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@ds0.me>

SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr