LLVM assert "May only branch on boolean predicates!" with bang predicate

[jruderman]

fn my_err(s: str) -> ! { log_err s; fail; }
fn main() { if my_err("bye") { } }

Assertion failed: (getCondition()->getType()->isIntegerTy(1) && "May only branch on boolean predicates!"), function AssertOK, file Instructions.cpp, line 669.