[X86][Atom] Fix integer shuffles uops, latency and throughput

The MMX pack/unpck shuffles don't need an override - they have the same behaviour as other shuffles (Port0 only).
The SSE pslldq/psrldq shuffles don't need an override - they have the same behaviour as other shuffles (Port0 only).
The SSE pshufb shuffles use 4uops (+1 load).

Noticed the pslldq/psrldq issue while trying to improve reduction costs via the D103695 helper script, and fixed the others while reviewing. Confirmed with Intel AoM / Agner / InstLatX64.