[x86] Make the x86 PACKSSWB, PACKSSDW, PACKUSWB, and PACKUSDW
instructions available as synthetic SDNodes PACKSS and PACKUS that will
select to the correct instruction variants based on the return type.
This allows us to use these rather important instructions when lowering
vector shuffles.

Also moves the relevant instruction definitions to be split out from
the fully generic multiclasses to allow them to match these new SDNodes
in the same way that the UNPCK instructions do.

No functionality should actually be changed here.