[BranchProbabilityInfo] Simplify getEdgeProbability (NFC)

The patch simplifies BranchProbabilityInfo::getEdgeProbability by
handling two cases separately, depending on whether we have edge
probabilities.

- If we have edge probabilities, then add up probabilities for
  successors being equal to Dst.

- Otherwise, return the number of ocurrences divided by the total
  number of successors.

Differential Revision: https://reviews.llvm.org/D90980