Incorrect pattern DynamicIotaOpToBroadcast

[christopherbate]

What happened?

// RUN: stablehlo-opt %s -stablehlo-aggressive-simplification

func.func @dynamic_iota_broadcast_second(%arg0 : tensor<2xi32>) -> tensor<5x?xi32> {
  %0 = "stablehlo.dynamic_iota"(%arg0) <{iota_dimension = 1 : i64}> : (tensor<2xi32>) -> tensor<5x?xi32>
  func.return %0 : tensor<5x?xi32>
}

Pattern DynamicIotaOpToBroadcast assumes arith.index_cast can be used even if operand and result types are both not index, but this is not true (perhaps it was at the time the MHLO code was written).

Anyways, it seems like arith.index_cast shouldn't be required anymore.

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