Rewrite OpStats to use llvm formatting utilities.

Example Output:

Operations encountered:
-----------------------
      addf                  , 11
      constant              , 4
      return                , 19
      some_op               , 1
   tf.AvgPool               , 3
   tf.DepthwiseConv2dNative , 3
   tf.FusedBatchNorm        , 2
  tfl.add                   , 7
  tfl.average_pool_2d       , 1
  tfl.leaky_relu            , 1

PiperOrigin-RevId: 229937190