[X86][SSE] getFauxShuffle - ignore undemanded sources for PACKSS/PACKUS faux shuffles

If we don't care about an entire LHS/RHS of the PACK op, then can just treat it the same as undef (we don't care if it saturates) and is safe to treat as a shuffle.

This can happen if we attempt to decode as a faux shuffle before SimplifyDemandedVectorElts has been called on the PACK which should replace the source with UNDEF entirely.