C#: Refactor last predicate

- Split up the `last` predicate into a non-recursive part `lastNonRec` and a recursive
  part `last`.
- Almost all syntactic constructs have a very simple `last` definition; a set of
  disjuncts with exactly one recursive call -- those are defined in `lastNonRec`.
- `try` statements and (last) `catch` clauses require multiple recursive calls in
  the same disjunct, and are therefore handled in the `last` predicate (as before).
- The benefit is that we only need to take care of the join order in the recursive
  call (for non-`try`/`catch` statements) in one place (the predicate `lastRec`),
  so we can get rid of many `nomagic`'ed `last`-specialisations.

Author: hvitved