[X86][AVX] Fold PACK(LOSUBVECTOR(SHUFFLE(X)),HISUBVECTOR(SHUFFLE(X))) -> SHUFFLE...

[X86][AVX] Fold PACK(LOSUBVECTOR(SHUFFLE(X)),HISUBVECTOR(SHUFFLE(X))) -> SHUFFLE(PACK(LOSUBVECTOR(X),HISUBVECTOR(X)))

Using PACK for truncations leaves us with intermediate shuffles that can be tricky to remove while the truncation tree is being formed.

This fold helps pull out the PERMQ case which is one of the most common, avoiding some costly lane-crossing shuffles.

A future patch will begin adding more general shuffle folding, which we should be able to use for HADD/HSUB as well.