sim: Evaluate all cells once

[povik]

If a cell starts with undefined inputs and the value of those inputs is never changed, sim code will never evaluate that cell. This means its output will be left at undefined, but in some edge cases such cell can evaluate to a defined value. To address those cases evaluate all cells at least once.

[povik]

One way this can be hit in practice is when some of the memory_* passes emit $eq cells comparing zero-sized signals, and those are not removed by opt_expr before sim is ran.