literal matching: separate path for pure literal patterns

src/parser/shortcut_literal.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2018, Intel Corporation
+ * Copyright (c) 2015-2019, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -185,6 +185,7 @@ bool shortcutLiteral(NG &ng, const ParsedExpression &pe) {
         return false;
     }
 
+    vis.lit.set_pure();
     const ue2_literal &lit = vis.lit;
 
     if (lit.empty()) {