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/compress/collapse_vars.js

@@ -1573,6 +1573,7 @@ var_side_effects_3: {
     options = {
         collapse_vars: true,
         pure_getters: true,
+        unsafe: true,
     }
     input: {
         var print = console.log.bind(console);