[X86][AVX] combineVectorSignBitsTruncation - use PACKSS/PACKUS in more AVX cases

AVX512 has fast truncation ops, but if the truncation source is a concatenation of subvectors then its likely that we can use PACK more efficiently.

This is only guaranteed to work for truncations to 128/256-bit vectors as the PACK works across 128-bit sub-lanes, for now I've just disabled 512-bit truncation cases but we need to get them working eventually for D61129.