Verify type checking of if-statement conditions

[powerboat9]

          LGTM i wonder if it is infact a coercion/cast-site in rustc for this case lets go with unify for now

Originally posted by @philberty in #2596 (review)

[tamaroning]

it seems that if-conditions are unification cites. coercions do not happen here.
https://rust-lang.github.io/rfcs/0401-coercions.html#coercions