After `memory_map` maps memories to flip-flops we need to let `opt`
remove undef muxes, otherwise we block enable/reset signal inference by
`opt_dff` which is in detriment to QoR.
`dfflegalize` will emit NOT gates to drive control signals on flip-flops
when mapping to supported flip-flop polarities. Typically in a design
this will produce a number of NOT gates driven by the same signal. For
one reason or another ABC doesn't fully cancel this redundancy during
LUT mapping. Insert an explicit `opt_merge` pass to improve synthesis
QoR.
