opt_share - merge mutually exclusive cells of the same type that share an input signal

    opt_share [selection]

This pass identifies mutually exclusive cells of the same type that:
    (a) share an input signal,
    (b) drive the same $mux, $_MUX_, or $pmux multiplexing cell,

allowing the cell to be merged and the multiplexer to be moved from
multiplexing its output to multiplexing the non-shared input signals.