Preserve labels when removing return statements

regression/goto-analyzer/label/main.c

@@ -0,0 +1,8 @@
+int main()
+{
+  goto ERROR;
+
+  return 0;
+ERROR:
+  return 1;
+}

regression/goto-analyzer/label/test.desc

@@ -0,0 +1,12 @@
+CORE
+main.c
+--show-goto-functions --error-label ERROR
+Labels: ERROR$
+ASSIGN main#return_value := 1$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+This test ensures that removal of return values (confirmed by the existence of
+the ASSIGN instruction) preserves labels attached to the return statement.

src/goto-programs/remove_returns.cpp

@@ -128,8 +128,10 @@ void remove_returnst::replace_returns(
         code_assignt assignment(*return_symbol, instruction.return_value());
 
         // now turn the `return' into `assignment'
+        auto labels = std::move(instruction.labels);
         instruction = goto_programt::make_assignment(
           assignment, instruction.source_location());
+        instruction.labels = std::move(labels);
       }
       else
         instruction.turn_into_skip();