[NFC] fix getOperandNo in PredIterator.

[simoll]

This fixes the following build error (c++ (GCC) 8.1.0):

llvm/include/llvm/IR/CFG.h:83:15: error: ‘const class llvm::Value::user_iterator_impl<const llvm::User>’ has no member named ‘getOperandNo’
     return It.getOperandNo();
            ~~~^~~~~~~~~~~~

[sunfish]

This looks right to me, but just to make sure I understand, has this been broken since r228400?

include/llvm/IR/CFG.h

public:
 
   inline Self operator++(int) { // Postincrement
     Self tmp = *this; ++*this; return tmp;
   }
 
   /// getOperandNo - Return the operand number in the predecessor's
   /// terminator of the successor.
   unsigned getOperandNo() const {
-    return It.getOperandNo();
+    return It.getUse().getOperandNo();
   }
 
   /// getUse - Return the operand Use in the predecessor's terminator
   /// of the successor.
   Use &getUse() const {
     return It.getUse();
   }
 };