[X86][AVX] Attempt to fold PACK(SHUFFLE(X,Y),SHUFFLE(X,Y)) -> SHUFFLE(PACK(X,Y)).

Truncations lowered as shuffles of multiple (concatenated) vectors often leave us with lane-crossing shuffles that feed a PACKSS/PACKUS, if both shuffles are fed from the same 2 vector sources, then we can PACK the sources directly and shuffle the result instead.

This is currently limited to whole i128 lanes in a 256-bit vector, but we can extend this if the need arises (but I'm not seeing many examples in real world code).