Solution:
- Go to Settings
- Click Apps
- Click
- Gear Icon
- Click Default Apps
- Click Assist & voice input
- Click Assist app.
- Changing that to None instead of Google App puts the dialer shortcut back on the lock screen, but it also disables long pressing the home button to launch the Google App/Now.
–jeroen
Source: Lock screen dialer shortcut gone in 6.0 – Android Forums at AndroidCentral.com