Optimization Passes for `dynamic_gather`

```mlir
module {
func.func @main(%arg0: tensor<6x6xf64>) -> tensor<6x6xf64> {
%cst = stablehlo.constant dense<1.000000e+00> : tensor<6x6xf64>
%cst_0 = stablehlo.constant dense<0.000000e+00> : tensor<6x6xf64>
%c = stablehlo.constant dense<[[1, 0], [2, 1], [3, 2], [4, 3], [5, 4], [0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 5], [0, 1], [1, 2], [2, 3], [3, 4], [4, 5]]> : tensor<16x2xi64>
%c_1 = stablehlo.constant dense<[[0, 1], [1, 2], [2, 3], [3, 4], [4, 5]]> : tensor<5x2xi64>
%c_2 = stablehlo.constant dense<[[0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 5]]> : tensor<6x2xi64>
%c_3 = stablehlo.constant dense<1> : tensor<2xi64>
%c_4 = stablehlo.constant dense<[[1, 0], [2, 1], [3, 2], [4, 3], [5, 4]]> : tensor<5x2xi64>
%0 = stablehlo.transpose %arg0, dims = [1, 0] : (tensor<6x6xf64>) -> tensor<6x6xf64>
%1 = "stablehlo.dynamic_gather"(%0, %c_4, %c_3) <{dimension_numbers = #stablehlo.gather}> : (tensor<6x6xf64>, tensor<5x2xi64>, tensor<2xi64>) -> tensor<5xf64>
%2 = "stablehlo.dynamic_gather"(%0, %c_2, %c_3) <{dimension_numbers = #stablehlo.gather}> : (tensor<6x6xf64>, tensor<6x2xi64>, tensor<2xi64>) -> tensor<6xf64>
%3 = "stablehlo.dynamic_gather"(%0, %c_1, %c_3) <{dimension_numbers = #stablehlo.gather}> : (tensor<6x6xf64>, tensor<5x2xi64>, tensor<2xi64>) -> tensor<5xf64>
%4 = stablehlo.concatenate %1, %2, %3, dim = 0 : (tensor<5xf64>, tensor<6xf64>, tensor<5xf64>) -> tensor<16xf64>
%5 = "stablehlo.scatter"(%cst_0, %c, %4) <{scatter_dimension_numbers = #stablehlo.scatter}> ({
^bb0(%arg1: tensor, %arg2: tensor):
stablehlo.return %arg2 : tensor
}) : (tensor<6x6xf64>, tensor<16x2xi64>, tensor<16xf64>) -> tensor<6x6xf64>
%6 = stablehlo.add %5, %cst : tensor<6x6xf64>
%7 = stablehlo.transpose %6, dims = [1, 0] : (tensor<6x6xf64>) -> tensor<6x6xf64>
return %7 : tensor<6x6xf64>
}
}
```

This comes from `fn(x) = Tridiagonal(x) .+ 1`.

Essentially if we fuse the `dynamic_gather`s into `dynamic_gather` + `slice`, then the `slice + concatenate` will get eliminated by another pass.

(In this particular case even the `dynamic_gather` + `scatter` can be eliminated, but that is probably much harder to write a pass for)