clk2fflogic - convert clocked FFs to generic $ff cells

yosys> help clk2fflogic
clk2fflogic [options] [selection]
This command replaces clocked flip-flops with generic $ff cells that use the
implicit global clock. This is useful for formal verification of designs with
multiple clocks.

This pass assumes negative hold time for the async FF inputs. For example when
a reset deasserts with the clock edge, then the FF output will still drive the
reset value in the next cycle regardless of the data-in value at the time of
the clock edge.
Do not automatically run 'chformal -lower' to lower $check cells.