i965/nir/vec4: Mark as unreachable ops that should be already lowered

i965/nir/vec4: Mark as unreachable ops that should be already lowered

NIR ALU operations:
   * nir_op_fabs
   * nir_op_iabs
   * nir_op_fneg
   * nir_op_ineg
   * nir_op_fsat
        should be lowered by lower_source mods

   * nir_op_fdiv
        should be lowered in the compiler by DIV_TO_MUL_RCP.

   * nir_op_fmod
        should be lowered in the compiler by MOD_TO_FLOOR.

   * nir_op_fsub
   * nir_op_isub
        should be handled by ir_sub_to_add_neg.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>