This fixes the edge case we had with variables pointing to dynamic
blocks, which forced us to convert basically *all* dynamic blocks to
DeadBlock when deallocating them.
We now don't run dynamic blocks through InterpState::deallocate() but
instead add them to a DeadAllocations list when they are deallocated but
still have pointers.
As a consequence, not all blocks with Block::IsDead = true are
DeadBlocks.
