Following on the plan of isolating the compiler/mlir/hlo directory. PiperOrigin-RevId: 320212018
This is part of the current refactoring of the HLO related dialect. `xla_hlo` will be reintroduced in a new form later. PiperOrigin-RevId: 319916753
Also add a localized `mlir-hlo-opt` binary for the testing of tensorflow/compiler/mlir/hlo/... ; this directory is intended to be self-contained and depend only on MLIR. PiperOrigin-RevId: 319878984