DxilPreparePass: down-convert equivalent new barrier ops for pre-SM 6.8 shader models.

[tex3d]

DxilPreparePass: down-convert equivalent new barrier ops for pre-SM 6.8 shader models. Test that new HLSL intrinsic with pre-SM 6.8 produces equivalent original barrier DXIL op. This must be done at the same time as the HLOperationLower task, since it's required to restore the original DXIL op for prior shader models.