Optimize Gather/Scatter into reshape with slice

The pre shape optimization IR is https://gist.github.com/wsmoses/40ec1e499cdf79ee0e65f1e2f77b74a8

We emit a bunch of gather/scatters that are needless. We should optimize them into slice/dynamic_update slice.

The reason they are emitted as gather/scatter is because during raising the tensor is 1d so it's not purely affine. However, clearly here the memory is two-dimensional so if we reshape it first to a 2d tensor, we can use a slice/dynamic update slice.

A walkthrough of the gather-scatter pattern is here: https://github.com/EnzymeAD/Enzyme-JAX/blob/8e83bb047e01f2f0499b5a0680405ff029330436/test/lit_tests/raising/affine_to_stablehlo_scatgath.mlir#L116

A sample optimization for gather is here: https://github.com/EnzymeAD/Enzyme-JAX/pull/1038/files