Use DenseIndex in the MeanReducer to avoid overflows when processing very large tensors.

unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h

diff --git a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
index 44dc2d7..33cd003 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
+++ b/unsupported/Eigen/CXX11/src/Tensor/TensorFunctors.h
@@ -158,8 +158,8 @@
   }
 
   protected:
-    int scalarCount_;
-    int packetCount_;
+    DenseIndex scalarCount_;
+    DenseIndex packetCount_;
 };
 
 template <typename T> struct MaxReducer