[X86] Preserve undef information when creating a punpckl/hbw from a v16i8...

[X86] Preserve undef information when creating a punpckl/hbw from a v16i8 where all the even or odd elements are undef.

Previously if V2 was unused we ended up using V1 for both inputs as part of the code that follows the new code. By using lowerVectorShuffleWithUNPCK we keep the undef nature of V2 in the output.

As near as I can tell this makes v16i8 behavior consistent with every other VT now.

This does mean that we give the register allocator freedom to fill in random registers now and create false dependencies. But like I said we're already doing that for other types.

llvm-svn: 347296