Add support for CollectiveBroadcast in MHLO

[ghpvnist]

We recently integrated collective_broadcast into the StableHLO spec in #1856. This change should also be migrated to MHLO.

This would involve updating the following to match StableHLO:

• https://github.com/openxla/xla/blob/main/xla/mlir_hlo/mhlo/IR/hlo_ops_attrs.td
• https://github.com/openxla/xla/blob/main/xla/mlir_hlo/mhlo/IR/hlo_ops.cc
• https://github.com/openxla/xla/blob/main/xla/mlir_hlo/mhlo/IR/hlo_ops.td
• https://github.com/openxla/xla/blob/main/xla/mlir_hlo/tests/Dialect/mhlo/hlo-legalize-to-stablehlo.mlir
• https://github.com/openxla/xla/blob/main/xla/mlir_hlo/tests/Dialect/mhlo/stablehlo-legalize-to-hlo.mlir

[ghpvnist]

Also need to update the StablehloOps.td (and hlo_ops.td) file to mark the op as Pure.

[ghpvnist]

Integrate is merged in openxla/xla@960438e.