Add differentiation support for non-active `try_apply` SIL instructions.
Notable pullback generation changes:
* Original basic blocks are now visited in a different order:
* starting from the original basic block, all its predecessors
* are visited in a breadth-first search order. This ensures that
* all successors of any block are visited before the block itself.
Resolves TF-433.
Support differentiation of `is` and `as?` operators.
These operators lower to branching cast SIL instructions, requiring control
flow differentiation support.
Resolves SR-12898.
