Simple expression if fails

```fstar
fn bug (b : bool)
(i2 i3 : unit -> fn returns UInt32.t)
returns UInt32.t
{
let y = (if b then i2 () else i3 ());
y;
}
```
```
- Tactic failed
- Cannot prove:
match b with
| true ->
exists* (__anf0: UInt32.t). with_pure (_posth == __anf0) (fun _ -> emp)
| false ->
with_pure (b == false)
(fun _ ->
exists* (__anf0: UInt32.t).
with_pure (_posth == __anf0) (fun _ -> emp))
- In the context:

Start by reproducing the provided F* function `bug` and inspect the tactic failure and generated proof obligation. Trace the simple `if` expression through the relevant tactic or verification entry point. Done means this example verifies without the reported unprovable obligation.

Written by the indexing model from the issue text.