prune the constant branches

[TomonobuTsujikawa]

This PR tends to optimize the tflite model converted from nnabla, which precompute the branches with completed constant inputs.
There are 6 changes in this PR:

• Precompute the constant branches
• Remove Split and slice operator if the shape of input and output are same
• Replace Split with slice if the model wants to be converted to quant model
• Replace PReLU with a combination of operators if the model wants to be converted to quant model
• Add the support of TRANSPOSE_CONV operator for quant model
• Update the support of pb to pb optimization to remove redundant transpose operators (version up to tf2.x)