Bug 646597 - Remove unneeded #ifdef DEBUG: use DebugOnly instead; r=bhackett

Assignment to a DebugOnly variable results in a call to either the empty
'operator =' or to an empty implicit 1-arg constructor and empty copy
constructor when in non-debug mode.  Either of these get inlined and
eliminated as dead code above -00, so there is no benefit to skipping
this assignment for non-debug builds.

js/src/jsanalyze.cpp

@@ -905,9 +905,7 @@ ScriptAnalysis::analyzeLifetimes(JSContext *cx)
                             ensureVariable(lifetimes[i], startOffset - 1);
                     }
 
-#ifdef DEBUG
                     found = true;
-#endif
                     break;
                 }
             }

mfbt/Util.h

@@ -174,7 +174,7 @@ struct DebugOnly
 #else
     DebugOnly() {}
     DebugOnly(const T&) {}
-    DebugOnly& operator=(const T&) {}   
+    DebugOnly& operator=(const T&) { return *this; }
 #endif
 
     /*