[X86] ComputeNumSignBitsForTargetNode - attempt to recognise...

[X86] ComputeNumSignBitsForTargetNode - attempt to recognise PACKSSDW(PACKSSDW(X,Y),PACKSSDW(Z,W)) patterns

These are often used when we're packing vXi64 comparison results, but we don't have PACKSSQD so have to bitcast, which doesn't work well with num sign bits value tracking.