x86: fix debug trace output for mwait

When running with the Exec flag, the mwait instruction attempted
to print out its source registers, which were never actually
initialized. This led to sporadic assertion failures when the
value stored there was invalid.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

src/arch/x86/isa/formats/monitor_mwait.isa

@@ -112,9 +112,9 @@ output decoder {{
     {
         std::stringstream response;
 
+        // Although mwait could take hints from eax and ecx, the _srcRegIdx
+        // is not set, and thus should not be printed here
         printMnemonic(response, mnemonic);
-        ccprintf(response, " ");
-        printReg(response, _srcRegIdx[0], machInst.opSize);
         return response.str();
     }
 }};