R600/SI: Handle i64 sub.

We can handle it the same way as add

llvm-svn: 211514
diff --git a/llvm/lib/Target/R600/R600ISelLowering.cpp b/llvm/lib/Target/R600/R600ISelLowering.cpp
index fda1ac7..8c08326 100644
--- a/llvm/lib/Target/R600/R600ISelLowering.cpp
+++ b/llvm/lib/Target/R600/R600ISelLowering.cpp
@@ -152,6 +152,8 @@
   setTargetDAGCombine(ISD::SELECT_CC);
   setTargetDAGCombine(ISD::INSERT_VECTOR_ELT);
 
+  setOperationAction(ISD::SUB, MVT::i64, Expand);
+
   // These should be replaced by UDVIREM, but it does not happen automatically
   // during Type Legalization
   setOperationAction(ISD::UDIV, MVT::i64, Custom);