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/iew_impl.hh

@@ -381,7 +381,7 @@ template <class Impl>
 bool
 DefaultIEW<Impl>::isDrained() const
 {
-    bool drained(ldstQueue.isDrained());
+    bool drained = ldstQueue.isDrained() && instQueue.isDrained();
 
     for (ThreadID tid = 0; tid < numThreads; tid++) {
         if (!insts[tid].empty()) {