DUS of select of slice [which is noop to dus], and actual val -> select(oldval, DUS(...))

```
%0 = stablehlo.slice %arg20 [0:20, 0:6144, 8:12280] : (tensor<20x6144x12288xf64>) -> tensor<20x6144x12272xf64> loc(8 : i64)
%16 = stablehlo.slice %arg20 [7:13, 7:6137, 8:12280] : (tensor<20x6144x12288xf64>) -> tensor<6x6130x12272xf64> loc(#loc81)
%32 = stablehlo.dynamic_update_slice %29, %12, %c_6, %c_3, %c_5 : (tensor<6x6130x12272xf64>, tensor<4x1x12272xf64>, tensor, tensor, tensor) -> tensor<6x6130x12272xf64> loc(#loc356)
%40 = stablehlo.select %39, %16, %32 : tensor, tensor<6x6130x12272xf64> loc(#loc81)
%46 = stablehlo.dynamic_update_slice %0, %40, %c_2, %c_2, %c_5 : (tensor<20x6144x12272xf64>, tensor<6x6130x12272xf64>, tensor, tensor, tensor) -> tensor<20x6144x12272xf64> loc(#loc81)
```

cc @ftynse