He'd still get ktfo, even though Froch's defense isnt great its not as bad as Khan's even if Khan had a granite chin he'd get hit so much he'd eventually get stopped
Khan needs a brain more than a chin. Khan's biggest problem is that once the opponent times him and figures him out he can't change things up. It's exactly the same problem Zab Judah has. Speed and flashy boxing with no ring smarts underpinning it.
Nigel Benn. He was a beast in the ring but with an iron chin on top of that he would have been scary. A relentless punching machine who you couldn't hurt.