-Zvalidate-mir should check that `SwitchInt` values are valid for their type

Hypothetically we could have a

```rust
SwitchInt {
    switch_ty: u8,
    values: [1, 2, 500],
    ...
}
```

we expect this not to happen, so this is an invariant of MIR that we can check

cc @rust-lang/wg-mir-opt @RalfJung 