[X86][AVX] combineVectorSignBitsTruncation - avoid complex vXi64->vXi32 PACKSS...

[X86][AVX] combineVectorSignBitsTruncation - avoid complex vXi64->vXi32 PACKSS truncations (PR45794)

Unless we're truncating an 'all-bits' result, using PACKSS for vXi64->vXi32 truncation causes problems with later combines as ComputeNumSignBits struggles to see through BITCASTs to smaller types. If we don't use PACKSS in these cases then we fallback to shuffles which are usually just as good.