opt_lut - optimize LUT cells

yosys> help opt_lut

opt_lut [options] [selection]

This pass combines cascaded $lut cells with unused inputs.

        -tech ice40
        treat the design as a LUT-mapped circuit for the iCE40 architecture
        and preserve connections to SB_CARRY as appropriate

-limit N

only perform the first N combines, then stop. useful for debugging.