[X86][AVX2] Recognise 256-bit truncation shuffles and mask 256-bit source

For v8i16 shuffle patterns that are lowered with AND+PACKUS, check to see if the sources are from a 256-bit vector and perform the masking using BLENDW at the 256-bit level.

With the test changes we can see more examples of duplicate XMM/YMM zero vectors (PR26018) :(