message: Implement IfOp verification
  This would also make the CallOp and ExtractElementOp invocations from eliminateIfOp function always valid and removes the need for error handling.
  Also, verify TensorFlowOp trait.
diff: @@ -408,10 +408,16 @@ public:
  TensorType() = default;
  /* implicit */ TensorType(Type::ImplType *ptr);
  + /// Return true if the specified element type is a TensorFlow type that is ok
  + /// in a tensor.
  + static bool isValidTFElementType(Type type) {
  + return type.isa<FloatType>() || type.isa<IntegerType>() ||
  + type.isa<OtherType>();
  + }
  +
  /// Return true if the specified element type is ok in a tensor.
  static bool isValidElementType(Type type) {
  - return type.isa<FloatType>() || type.isa<VectorType>() ||
  - type.isa<IntegerType>() || type.isa<OtherType>() ||
  + return isValidTFElementType(type) || type.isa<VectorType>() ||
  type.isa<IndexType>();
  }
  