Add FIXME comments with suggestions to improve test case maintenance.

Author: atrick

include/swift/SIL/SILCloner.h

@@ -614,6 +614,10 @@ void SILCloner<ImplClass>::visitBlocksDepthFirst(
   assert(preorderBlocks.empty());
 
   // First clone the CFG region.
+  //
+  // FIXME: Add reverse iteration to SILSuccessor, then convert this to an RPOT
+  // traversal. We would prefer to keep CFG regions in RPO order, and this would
+  // be more scalable for functions with many large switches.
   SmallVector<SILBasicBlock *, 8> dfsWorklist(1, startBB);
   while (!dfsWorklist.empty()) {
     auto *BB = dfsWorklist.pop_back_val();

lib/Serialization/SerializeSIL.cpp

@@ -411,6 +411,10 @@ void SILSerializer::writeSILFunction(const SILFunction &F, bool DeclOnly) {
   BasicBlockMap.clear();
   // Assign a value ID to each SILInstruction that has value and to each basic
   // block argument.
+  //
+  // FIXME: Add reverse iteration to SILSuccessor and convert this to a "stable"
+  // RPO order. Currently, the serializer inverts the order of successors each
+  // time they are processed.
   unsigned ValueID = 0;
   llvm::ReversePostOrderTraversal<SILFunction *> RPOT(
       const_cast<SILFunction *>(&F));