Add CastBranchInstBase::SuccessorPath for easier factoring.

Code that handles checked_cast_br can not be factored easily without
coding separate paths for each successor.

Author: atrick

include/swift/SIL/SILInstruction.h

@@ -8767,10 +8767,13 @@ template <typename BaseTy> class CastBranchInstBase : public BaseTy {
 
   TermInst::SuccessorListTy getSuccessors() { return DestBBs; }
 
-  SILBasicBlock *getSuccessBB() { return DestBBs[0]; }
-  const SILBasicBlock *getSuccessBB() const { return DestBBs[0]; }
-  SILBasicBlock *getFailureBB() { return DestBBs[1]; }
-  const SILBasicBlock *getFailureBB() const { return DestBBs[1]; }
+  // Enumerate the successor indices
+  enum SuccessorPath { SuccessIdx = 0, FailIdx = 1};
+
+  SILBasicBlock *getSuccessBB() { return DestBBs[SuccessIdx]; }
+  const SILBasicBlock *getSuccessBB() const { return DestBBs[SuccessIdx]; }
+  SILBasicBlock *getFailureBB() { return DestBBs[FailIdx]; }
+  const SILBasicBlock *getFailureBB() const { return DestBBs[FailIdx]; }
 
   /// The number of times the True branch was executed
   ProfileCounter getTrueBBCount() const { return DestBBs[0].getCount(); }