[X86] LowerRotate - recognise hidden ROTR patterns for better vXi8 codegen

Check for a hidden ISD::ROTR (rotl(sub(0,x))) - vXi8 lowering can handle both (its always beneficial for splats, but otherwise only if we have VPTERNLOG).

We currently hit infinite loops in TargetLowering::expandROT if we set ISD::ROTR to custom, which needs addressing before we extend this much further.