- introduces a simplify pass, which finds such patterns and replaces it
with the equivalent `reduce-scatter`
- promotes the test-pass `test-shard-optimizations` to a proper pass and adds
- folding allgather+allslice into reduce_scatter
- sanitizes the `shard.reduce_scatter` op
- adds a new `mpi.reduce_scatter_block` op
- lowers `shard.reduce_scatter` to MPI
- lowers `mpi-reduce_scatter_block` to llvm
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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