fix chained evaluation (#1610)

`reduce_vars` enables substitution of variables but did not clone the value's `AST_Node`.

This confuses `collapse_vars` and result in invalid AST and subsequent crash.

fixes #1609

lib/compress.js

@@ -3461,7 +3461,7 @@ merge(Compressor.prototype, {
                     }
                 }
                 if (d.should_replace) {
-                    return d.should_replace;
+                    return d.should_replace.clone(true);
                 }
             }
         }