improve pure_getters (#1786)

- property access to `null` & `undefined` always has side effects
- utilise `reduce_vars` to determine safe property access
- may-be cases treated as side effects unless `unsafe`

test/ufuzz.json

@@ -35,7 +35,6 @@
         "compress": {
             "keep_fargs": false,
             "passes": 3,
-            "pure_getters": true,
             "warnings": false
         }
     }