cpu: Add drain check functionality to IEW

IEW did not check the instQueue and memDepUnit to ensure
they were drained.  This caused issues when drainSanityCheck()
did check those structures after asserting IEW was drained.

src/cpu/o3/inst_queue_impl.hh

@@ -440,6 +440,17 @@ InstructionQueue<Impl>::setTimeBuffer(TimeBuffer<TimeStruct> *tb_ptr)
     fromCommit = timeBuffer->getWire(-commitToIEWDelay);
 }
 
+template <class Impl>
+bool
+InstructionQueue<Impl>::isDrained() const
+{
+    bool drained = dependGraph.empty() && instsToExecute.empty();
+    for (ThreadID tid = 0; tid < numThreads; ++tid)
+        drained = drained && memDepUnit[tid].isDrained();
+
+    return drained;
+}
+
 template <class Impl>
 void
 InstructionQueue<Impl>::drainSanityCheck() const