Changed the behavior of expr_linearity to consider min/max of linear expressions as linear. This may seem controversial, but I guess we can say that the definition of linearity is taken piecewise. The need for this change is due to the fact that we want to check for branches that branch linearly in a variable without considering min/max as inducing branches.
Small but important bug fix, that was causing nested let nodes to be incorrectly rebuilt during branching.