[IR] Change access rights of PredIterator members

[slydiman]

Summary

These members were made private here https://github.com/llvm/llvm-project/commit/6177386b05ddaaee04160b153a41ed4edbd7bcaf without an explanation.
Our customers have an own implementation inherited from PredIterator with updated advancePastNonTerminators().
The access specifier protected looks resonable and safe here.

[RKSimon]

LGTM

llvm/include/llvm/IR/CFG.h

42

We usually inherit (CRTP style) from iterator_adaptor_base - not sure why we don't here.

llvm/include/llvm/IR/CFG.h

 class Instruction;
 class Use;
 
 //===----------------------------------------------------------------------===//
 // BasicBlock pred_iterator definition
 //===----------------------------------------------------------------------===//
 
 template <class Ptr, class USE_iterator> // Predecessor Iterator
 class PredIterator {
 public:
   using iterator_category = std::forward_iterator_tag;
   using value_type = Ptr;
   using difference_type = std::ptrdiff_t;
   using pointer = Ptr *;
   using reference = Ptr *;
 
-private:
+protected:
   using Self = PredIterator<Ptr, USE_iterator>;
   USE_iterator It;
 
   inline void advancePastNonTerminators() {
     // Loop to ignore non-terminator uses (for example BlockAddresses).
     while (!It.atEnd()) {
       if (auto *Inst = dyn_cast<Instruction>(*It))
         if (Inst->isTerminator())