Enable optimizer pipeline for DBC.

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 a7f3d7f8c4c6f3006486a53c33cf2fe8b52411a3..12a95f983fa9a1cc122c06eb947abf2cc313c9a2 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -8188,6 +8188,17 @@ class Environment : public ZoneAllocated {
   // from the copy.
   Environment* DeepCopy(Zone* zone, intptr_t length) const;
 
+#if defined(TARGET_ARCH_DBC)
+  // Return/ReturnTOS instruction drops incoming arguments so
+  // we have to drop outgoing arguments from the innermost environment.
+  // On all other architectures caller drops outgoing arguments itself
+  // hence the difference.
+  // Note: this method can only be used at the code generation stage because
+  // it mutates environment in unsafe way (e.g. does not update def-use
+  // chains).
+  void DropArguments(intptr_t argc);
+#endif
+
  private:
   friend class ShallowIterator;