[mlir][ArmSME] Add custom get_tile_id and cast ops

This patch adds three new custom ops to the ArmSME dialect:

  * arm_sme.get_tile_id - returns a scalar integer representing an SME
    "virtual tile" that is not in use.
  * arm_sme.cast_tile_to_vector - casts from a tile id to a 2-d scalable
    vector type, which represents an SME "virtual tile".
  * arm_sme.cast_vector_to_tile - casts from a 2-d scalable vector type,
    which represents an SME "virtual tile", to a tile id.

The 'arm_sme.get_tile_id' op currently only supports tile 0, a follow-up
patch will implement proper tile allocation. A further follow-up patch
will demonstrate load/store to/from ZA using these ops.

See the op descriptions for further details and examples.

Thanks to @paulwalker-arm and @awarzynski for helping drive this.

Reviewed By: awarzynski, dcaballe

Differential Revision: https://reviews.llvm.org/D154941