MIR SourceScopes stay around even when all their uses were removed.

[eddyb]

We already have a testcase, namely simplify_try, which replaces enough of the original body to make some SourceScopes unused.

However, despite no statements/terminators using those scopes anymore, they stick around.
The fact that they're gone is in itself a problem (debuginfo is lost, see comments on #66282), but we could argue that SourceScopes only used in unreachable code should still be removed, e.g.:

if false {
    let x = 123;
}

There is no reason to have debuginfo for x tracked anywhere once the x = 123; block is removed.

@rust-lang/wg-mir-opt