[X86][AVX] truncateVectorWithPACK - avoid bitcasted shuffles

truncateVectorWithPACK is often used in conjunction with ComputeNumSignBits which struggles when peeking through bitcasts.

This fix tries to avoid bitcast(shuffle(bitcast())) patterns in the 256-bit 64-bit sublane shuffles so we can still see through at least until lowering when the shuffles will need to be bitcasted to widen the shuffle type.

llvm-svn: 364401