Instead of a hash map mapping pairs of blocks and successor index to the
probability, store the probabilities as flat array and start indices
into this array in a per-block information vector.
Also drop value handles: no stored pointers => no stale pointers. If a
block is removed, the block number is not reused unless the function is
renumbered, and BPI doesn't support renumbering.
In case the same src BB targets to the same dest BB in different
conditions/edges, such as switch-cases, we should use
prob[SrcBB->SuccIndx] instead of prob[SrcBB->DstBB] to get probability.
