Set previous instruction pointers when building the graph.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 1e1ec251f842088789e7202228ab33589eb51013..43adb00447c0ee5a66f3784720004315fadd8d8c 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -352,6 +352,13 @@ class Instruction : public ZoneAllocated {
     next_ = instr;
   }
 
+  // Link together two instruction.
+  void LinkTo(Instruction* next) {
+    ASSERT(this != next);
+    this->set_next(next);
+    next->set_previous(this);
+  }
+
   // Removed this instruction from the graph.
   Instruction* RemoveFromGraph(bool return_previous = true);