[BranchProbabilityInfo] Simplify getEdgeProbability (NFC)

[kazu]

Summary

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.

[MaskRay]

LGTM.

llvm/lib/Analysis/BranchProbabilityInfo.cpp

BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
   return getEdgeProbability(Src, Dst.getSuccessorIndex());
 }
 
 /// Get the raw edge probability calculated for the block pair. This returns the
 /// sum of all raw edge probabilities from Src to Dst.
 BranchProbability
 BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
                                           const BasicBlock *Dst) const {
+  if (!Probs.count(std::make_pair(Src, 0)))
+    return BranchProbability(llvm::count(successors(Src), Dst), succ_size(Src));
+
   auto Prob = BranchProbability::getZero();
-  bool FoundProb = false;
-  uint32_t EdgeCount = 0;
   for (const_succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
-    if (*I == Dst) {
-      ++EdgeCount;
-      auto MapI = Probs.find(std::make_pair(Src, I.getSuccessorIndex()));
-      if (MapI != Probs.end()) {
-        FoundProb = true;
-        Prob += MapI->second;
-      }
-    }
-  return FoundProb ? Prob : BranchProbability(EdgeCount, succ_size(Src));
+    if (*I == Dst)
+      Prob += Probs.find(std::make_pair(Src, I.getSuccessorIndex()))->second;
+
+  return Prob;
 }
 
 /// Set the edge probability for all edges at once.
 void BranchProbabilityInfo::setEdgeProbability(
     const BasicBlock *Src, const SmallVectorImpl<BranchProbability> &Probs) {
   assert(Src->getTerminator()->getNumSuccessors() == Probs.size());
   eraseBlock(Src); // Erase stale data if any.
   if (Probs.size() == 0)