[MLIR][mesh] Mesh fixes (#124724)
A collection of fixes to the mesh dialect
- allow constants in sharding propagation/spmdization
- fixes to tensor replication (e.g. 0d tensors)
- improved canonicalization
- sharding propagation incorrectly generated too many ShardOps
New operation `mesh.GetShardOp` enables exchanging sharding information
(like on function boundaries)
