Bad range on single-branch if

This fails (rightly, since we are just admitting when x=0) but highlights the body of the then-branch as the error location. Some indication that the problem is in the else-branch would be good.

````fstar
assume val f : int -> vprop
```pulse
fn test (x:int)
requires f 0
ensures f x
{
();
();
if (x = 0) {
();
();
admit();
();
();
}
}
```
````

Start by running the Pulse example in the issue and inspect the diagnostic produced for the failing single-branch if. Trace the source-range handling for this example; done means the reported location indicates that the problem is in the else-branch rather than highlighting the then-branch body.

Written by the indexing model from the issue text.