I get PhDs are a helluva process, and achievement, so potentially derailing one is a bear of a choice to confront. But I have a question for all these people advising OP to stick their head in the sand and stomach this level of abuse from someone who is supposed to be in a mentor position.... What exactly makes any of you think this person will act fairly when it comes time to review OPs work and assign them their PhD? They could be putting up with this garbage for their work to amount to literally nothing. And what will they have to show for it? Oh yeah, learning how to roll over and say "hit me again Ike, and this time put some stank on it." That is seriously y'all's advice eh? Wow ... Just wow.... Milksops is all I gotta say about that.
OP it is ultimately up to you. However universities usually have a plethora of avenues for resolution. Talk to your student advisor (which you should have already btw), talk to other faculty, talk to the faculty supervisor, hell talk to the dean! Bring all the other students that have had this person disparage them with no provocation. Look up former students and see if they have had similar experiences. Point out to anyone in the higher ups (I forget how the command structure works at proper unis, but make stinks on desks of people that shouldn't have to deal with this. Don't be afraid to jump rungs on the ladder if the one you're at isn't listening) the money, time, and resources wasted, not to mention the mental and emotional stresses experienced by the students exposed to that treatment, will not benefit the universities image should the story of a professor (er whatever that supervisor is) calling their students "retard" get too much traction publicly. I imagine there's a handful of benefactors that would not like to hear what's going on at the school they are dumping money into.
Sure, if you want to wait to drop the bomb, that is entirely your call. In this day and age it's not unusual for young people to avoid the prospect of rocking the boat. But remember, the only thing needed for evil to flourish is for good to do nothing. If there is one thing we need to fear above the tyranny of evil men, it is the indifference of good men. Whatever you do, do not stay silent. Do it tomorrow, do it after you (hopefully) secure your PhD, just do it! To do nothing is being complacent with the abusive nature of this person. Being complacent is endorsing treating others that way. It will directly lead to others also being abused, disparaged, made to feel like lesser beings which no one deserves especially at a university particularly by a member of the faculty.
I understand if you are stressed, I understand if you are afraid of confrontation, I understand if you just want it to go away. All of those feelings are entirely natural, everyone who has ever needed to stand up to mistreatment has felt those feelings and more. But ignoring this behavior before is what lead to you to being treated this way. Embrace the disappointment, dissatisfaction, and anger towards those who fostered this absolute wretch of a human and allowed them to continue verbally attacking students, turn it into motivation to save future students from such treatment at the hands of someone who is supposed to be fostering and inspiring the next generation of educated individuals needed in the world.
You will be afraid, you will be scared, you will feel small and a loss of worth and unimportant. Most importantly.....you WILL be doing the right thing. Never loose sight of that. You're not alone. There are people who will help (try reddit, both the academia, and legal advice subs). You only need to seek them out.
My final bit of advice is to always keep this in your mind: no matter which avenue you choose, You CAN do this. And I believe in you. Now, go carpe that diem BY THE THROAT!!!