Fix false positive by Lint/UselessAssignment.

This code would wrongly produce a UselessAssignment on "myvar=0"
```
def useless_assignment_bug
  myvar = 0

  if cond
    myvar = nil               # 'myvar = 1' makes the bug go away
    bar if baz                # 'bar' kills the bug
  end

  myvar
end
```

because on_if did not return `node` and therefore this cop managed to
confuse another cop.

lib/rubocop/yast/track_variable_scope.rb

@@ -70,6 +70,8 @@ def on_if(node)
 
         # clean slate
         scope.clear
+
+        node
       end
 
       # def on_unless
@@ -91,39 +93,46 @@ def on_case(node)
 
         # clean slate
         scope.clear
+
+        node
       end
 
       def on_lvasgn(node)
         super
         name, value = * node
         return if value.nil? # and-asgn, or-asgn, resbody do this
         scope[name].nice = nice(value)
+        node
       end
 
       def on_and_asgn(node)
         super
         var, value = *node
         bool_op_asgn(var, value, :and)
+        node
       end
 
       def on_or_asgn(node)
         super
         var, value = *node
         bool_op_asgn(var, value, :or)
+        node
       end
 
-      def on_block(_node)
+      def on_block(node)
         # ignore body, clean slate
         scope.clear
+        node
       end
       alias_method :on_for, :on_block
 
-      def on_while(_node)
+      def on_while(node)
         # ignore both condition and body,
         # with a simplistic scope we cannot handle them
 
         # clean slate
         scope.clear
+        node
       end
       alias_method :on_until, :on_while
 
@@ -143,8 +152,10 @@ def on_rescue(node)
           process(r)
         end
         #  end
+        node
       rescue TooComplexToTranslateError
         warn "begin-rescue is too complex to translate due to a retry"
+        node
       end
 
       def on_resbody(node)
@@ -161,11 +172,12 @@ def on_resbody(node)
         super
       end
 
-      def on_ensure(_node)
+      def on_ensure(node)
         # (:ensure, guarded-code, ensuring-code)
         # guarded-code may be a :rescue or not
 
         scope.clear
+        node
       end
 
       def on_retry(_node)