[JIT] Enable branching on fixed column values

[chriseth]

We should be able to branch on fixed column values that are known but not concrete. This is currently not possible, because the branching code iterates over all variables in witgen.known_variables(), which just returns a reference to the HashSet of known variables.
Fixed column cell references are implicitly marked as known though. The main reason for that is that they are known from the start, but we do not really know for which row offsets they will later occur in the constraints.