In my case, I may compare this with “wanna” (want to) or “gotta” (got to).
It's faster to speak… But I just say this when I'm with friends, not with my boss…
Writing “t'as” is not correct in French, it's a bad part of what we call “une forme élidée”.
Post scriptum: I post it as an answer but I hope some specialist could explain better than I do. And it was too long to post it as a comment. I'm not fluent in English, I've got a technical English…
