Add the $anyinit cell and the formalff pass

These can be used to protect undefined flip-flop initialization values
from optimizations that are not sound for formal verification and can
help mapping all solver-provided values in witness traces for flows that
use different backends simultaneously.

kernel/celltypes.h

@@ -51,6 +51,7 @@ struct CellTypes
 
 		setup_internals();
 		setup_internals_mem();
+		setup_internals_anyinit();
 		setup_stdcells();
 		setup_stdcells_mem();
 	}
@@ -155,6 +156,11 @@ struct CellTypes
 		setup_type(ID($dlatchsr), {ID::EN, ID::SET, ID::CLR, ID::D}, {ID::Q});
 	}
 
+	void setup_internals_anyinit()
+	{
+		setup_type(ID($anyinit), {ID::D}, {ID::Q});
+	}
+
 	void setup_internals_mem()
 	{
 		setup_internals_ff();