[BOLT] Fix branch data for __builtin_unreachable().

Summary:
When we have a conditional branch past the end of function (a result
of a call to__builtin_unreachable()), we replace the branch with nop,
but keep branch information for validation purposes. If that branch
has a recorded profile we mistakenly create an additional successor
to a containing basic block (a 3rd successor).

Instead of adding the branch to FTBranches list we should be adding
to IgnoredBranches.

(cherry picked from FBD4912840)

bolt/BinaryFunction.cpp

@@ -1134,8 +1134,8 @@ void BinaryFunction::disassemble(ArrayRef<uint8_t> FunctionData) {
                            << " : replacing with nop.\n");
               BC.MIA->createNoop(Instruction);
               if (IsCondBranch) {
-                // Register FT branch for passing function profile validation.
-                FTBranches.emplace_back(Offset, Offset + Size);
+                // Register branch function profile validation.
+                IgnoredBranches.emplace_back(Offset, Offset + Size);
               }
               goto add_instruction;
             }
@@ -1888,6 +1888,7 @@ bool BinaryFunction::buildCFG() {
   clearList(OffsetToCFI);
   clearList(TakenBranches);
   clearList(FTBranches);
+  clearList(IgnoredBranches);
   clearList(LPToBBIndex);
   clearList(EntryOffsets);
 
@@ -1986,6 +1987,9 @@ void BinaryFunction::evaluateProfileData(const FuncBranchData &BranchData) {
   FunctionBranches.insert(FunctionBranches.end(),
                           FTBranches.begin(),
                           FTBranches.end());
+  FunctionBranches.insert(FunctionBranches.end(),
+                          IgnoredBranches.begin(),
+                          IgnoredBranches.end());
   std::sort(FunctionBranches.begin(), FunctionBranches.end());
 
   BranchListType DiffBranches; // Branches in profile without a match.

bolt/BinaryFunction.h

@@ -439,8 +439,9 @@ class BinaryFunction : public AddressRangesOwner {
   std::unordered_map<const MCSymbol *, BinaryBasicBlock *> LabelToBB;
 
   using BranchListType = std::vector<std::pair<uint32_t, uint32_t>>;
-  BranchListType TakenBranches; /// All local taken branches.
-  BranchListType FTBranches;    /// All fall-through branches.
+  BranchListType TakenBranches;       /// All local taken branches.
+  BranchListType FTBranches;          /// All fall-through branches.
+  BranchListType IgnoredBranches;     /// Branches ignored by CFG purposes.
 
   /// Storage for all landing pads and their corresponding invokes.
   using LandingPadsMapType = std::map<const MCSymbol *, std::vector<unsigned> >;