[PATCH] [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.

[Chandler Carruth]

Committed in r211332 with suggested tweaks and better commit log. Thanks!

================
Comment at: lib/Target/X86/X86InstrSSE.td:4421
@@ -4434,1 +4420,3 @@
 //===---------------------------------------------------------------------===//
+// SSE - Packed Integer Pack Instructions
+//===---------------------------------------------------------------------===//
----------------
Jim Grosbach wrote:
> Nipick. These define far more than just the SSE2 instructions. Perhaps update the comment(s) to better reflect modern reality?
Sure, will do.

http://reviews.llvm.org/D4016