opt_dff - perform DFF optimizations

    opt_dff [-nodffe] [-nosdff] [-keepdc] [-sat] [selection]

This pass converts flip-flops to a more suitable type by merging clock enables
and synchronous reset multiplexers, removing unused control inputs, or potentially
removes the flip-flop altogether, converting it to a constant driver.

        disables dff -> dffe conversion, and other transforms recognizing clock enable

        disables dff -> sdff conversion, and other transforms recognizing sync resets

        only enables clock enable recognition transform for obvious cases

        additionally invoke SAT solver to detect and remove flip-flops (with
        non-constant inputs) that can also be replaced with a constant driver

        some optimizations change the behavior of the circuit with respect to
        don't-care bits. for example in 'a+0' a single x-bit in 'a' will cause
        all result bits to be set to x. this behavior changes when 'a+0' is
        replaced by 'a'. the -keepdc option disables all such optimizations.