coverage: Unbox and simplify `bcb_filtered_successors`

This function already has access to the MIR body, so instead of taking a
reference to a terminator, it's simpler and easier to pass in a basic block
index.

There is no need to box the returned iterator if we instead add appropriate
lifetime captures, since `short_circuit_preorder` is now generic over the type
of iterator it expects.

We can also greatly simplify the function's implementation by observing that
the only difference between its two cases is whether we take all of a BB's
successors, or just the first one.

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