Comparison functions (e.g. isAfter()) not working with timezone dates

Date comparison functions like `isAfter()` do not work with `dayjs.tz` dates.

**Example**

```javascript
const dayjs = require('dayjs')
const utc = require('dayjs/plugin/utc')
const timezone = require('dayjs/plugin/timezone')

dayjs.extend(utc)
dayjs.extend(timezone)
dayjs.tz.setDefault('Europe/Berlin')

console.log('working as expected:')

let before = dayjs('2021-01-03T20:00:00+01:00')
let after = dayjs('2021-01-03T20:30:00+01:00') //30 minutes later

console.log('before:', before.format()) // => 2021-01-03T20:00:00+01:00
console.log('after:', after.format()) // => 2021-01-03T20:30:00+01:00
console.log('after.isAfter(before) ==>', after.isAfter(before)) // => true ✅
console.log('before.isAfter(after) ==>', before.isAfter(after)) // => false ✅

console.log('\n')

console.log('NOT working as expected:')

before = dayjs.tz('01.03.2021 21:00:00', 'DD.MM.YYYY HH:mm', 'Europe/Berlin')
after = dayjs.tz('01.03.2021 21:30:00', 'DD.MM.YYYY HH:mm', 'Europe/Berlin') //30 minutes later

console.log('before:', before.format()) // => 2021-01-03T20:00:00+01:00
console.log('after:', after.format()) // => 2021-01-03T20:30:00+01:00

console.log('after.isAfter(before) ==>', after.isAfter(before)) // => false ❌
console.log('before.isAfter(after) ==>', before.isAfter(after)) // => false ✅
```

**Information**
- Day.js Version: v1.8.34
- OS: MacOS 11.2.3
- Node.js
- Time zone: CET

Start by running the supplied Node.js reproduction with the utc and timezone plugins, then inspect the dayjs.tz entry point and comparison functions such as isAfter(). Done means timezone-created dates produce the expected true/false comparison results shown in the example.

Written by the indexing model from the issue text.