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.hh

@@ -145,6 +145,9 @@ class InstructionQueue
     /** Sets the global time buffer. */
     void setTimeBuffer(TimeBuffer<TimeStruct> *tb_ptr);
 
+    /** Determine if we are drained. */
+    bool isDrained() const;
+
     /** Perform sanity checks after a drain. */
     void drainSanityCheck() const;