One Hot Array Optimization Patterns

- [x] `scatter` with multiply (https://github.com/EnzymeAD/Enzyme-JAX/pull/1033)

```mlir
%cst_1 = stablehlo.constant 1 <.....>
%cst_0 = stablehlo.constant 0 <.....>

%materialized_one_hot_array = stablehlo.scatter(%cst_0, %cst_1)
... <- setindex essentially

%res = stablehlo.multiply %materialized_one_hot_array %b
```

convert this into a gather of indices of `%b` followed by a `scatter` of those indices

Start by locating the compiler pattern that recognizes the shown stablehlo.scatter followed by stablehlo.multiply sequence. Compare the existing scatter, gather, and setindex handling, then verify that the one-hot materialization is replaced by the requested gather-of-indices and scatter form without changing the result.

Written by the indexing model from the issue text.