sudo nvram boot-args="-x"
What will actually happen is that the Mac will enter an endless cycle of rebooting every time you enter your password to unlock the disk.
If you have done this, then the way to get out of it is to reset your PRAM like this:
Do a reboot, and immediately hit ⌥ ⌘ P R together (Option + Command + P + R), and hold them down until the Mac reboots again. Now it should boot up normally.
If you need to perform a Safe Boot, Apple say that you should disable FileVault 2 first. (I have not tried this.)