The BAthory Demos have growling on them , and they are from 1982-83...from the story I've heard, his PA was blown, and distorted his thrash-like yelling...he liked the sound so much, he started imitating it by growling...I'd say they were the first, and definitely the first to step down the path of what we call "extreme metal" today, followed closely by Celtic Frost, Hellhammer, etc.
As for clean/growl, if you count thrash vox as clean, then Testament did it in the 80's...there's a growl or two on New Order ('88) and more than enough on Practice what you Preach ('89)