[js] Handle functions with rest parameters in isObservableArgumentSlot.

js/src/jit/CompileInfo.h

@@ -487,7 +487,7 @@ class CompileInfo
 
         // Function.arguments can be used to access all arguments in non-strict
         // scripts, so we can't optimize out any arguments.
-        if ((hasArguments() || !script()->strict()) &&
+        if ((mayReadFrameArgsDirectly_ || !script()->strict()) &&
             firstArgSlot() <= slot && slot - firstArgSlot() < nargs())
         {
             return true;