concat reshape slice -> dus reshape

```
%1234 = stablehlo.slice %iterArg_334 [5:6, 0:1, 0:3056] : (tensor<6x1522x3056xf64>) -> tensor<1x1x3056xf64> loc(#loc)
%1214 = stablehlo.slice %203 [3:4, 0:1520, 0:3056] : (tensor<4x1520x3056xf64>) -> tensor<1x1520x3056xf64> loc(#loc4283)
%1235 = stablehlo.slice %iterArg_334 [5:6, 1521:1522, 0:3056] : (tensor<6x1522x3056xf64>) -> tensor<1x1x3056xf64> loc(#loc)

%1427 = stablehlo.reshape %1234 : (tensor<1x1x3056xf64>) -> tensor<1x3056xf64> loc(#loc4437)
%1428 = stablehlo.reshape %1214 : (tensor<1x1520x3056xf64>) -> tensor<1520x3056xf64> loc(#loc4437)
%1429 = stablehlo.reshape %1235 : (tensor<1x1x3056xf64>) -> tensor<1x3056xf64> loc(#loc4437)

%1430 = stablehlo.concatenate %1427, %1428, %1429, dim = 0 : (tensor<1x3056xf64>, tensor<1520x3056xf64>, tensor<1x3056xf64>) -> tensor<1522x3056xf64> loc(#loc4437)
```

Start by tracing how the compiler handles the supplied StableHLO slice, reshape, and concatenate sequence, then compare it with the requested DUS reshape form. No file, test, or entry point is named; done means the shown pattern is transformed correctly and covered by an appropriate regression test.

Written by the indexing model from the issue text.