Post-condition inference fails without rewrites_to

```fstar
module Bug455
open Pulse
#lang-pulse

fn bar ()
returns r: bool
{
true
}

fn foo () {
while (not (bar ()))
(* ^^^^^^^^^^^^
- Could not resolve all free variables in the proposition:
_posth == op_Negation ?__anf122
*)
invariant (emp)
{ () };
}
```

Start by reproducing the minimal Bug455 Pulse example and compare post-condition inference with and without rewrites_to. Trace the inference for the while condition and verify that the free-variable error no longer occurs when the issue's example is processed without rewrites_to.

Written by the indexing model from the issue text.