xilinx_dffopt - Xilinx: optimize FF control signal usage

yosys> help xilinx_dffopt

xilinx_dffopt [options] [selection]

Converts hardware clock enable and set/reset signals on FFs to emulation
using LUTs, if doing so would improve area.  Operates on post-techmap Xilinx
cells (LUT*, FD*).

-lut4

Assume a LUT4-based device (instead of a LUT6-based device).