ice40_opt - iCE40: perform simple optimizations

    ice40_opt [options] [selection]

This command executes the following script:

    do
        <ice40 specific optimizations>
        opt_expr -mux_undef -undriven [-full]
        opt_merge
        opt_rmdff
        opt_clean
    while <changed design>

When called with the option -unlut, this command will transform all already
mapped SB_LUT4 cells back to logic.