[GISel][NFC]: Make members of CombinerHelper accessible in derived classes

[dsanders]

LGTM

[aditya_nandakumar]

Thanks. r368248

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

 
 struct PreferredTuple {
   LLT Ty;                // The result type of the extend.
   unsigned ExtendOpcode; // G_ANYEXT/G_SEXT/G_ZEXT
   MachineInstr *MI;
 };
 
 class CombinerHelper {
+protected:
   MachineIRBuilder &Builder;
   MachineRegisterInfo &MRI;
   GISelChangeObserver &Observer;
   GISelKnownBits *KB;
 
 public:
   CombinerHelper(GISelChangeObserver &Observer, MachineIRBuilder &B,
                  GISelKnownBits *KB = nullptr);