[DSLX:TIv2] Overshifts should be allowed for parameterized types

If we make a parameterized routine, and do something like:

```
fn f(x: uN[N]) -> uN[N] {
(x >> 4) + x
}
```

with the current TIv2 warnings we'll flag an overshift if `N <=3`, but this routine is mathematically ok in that range, so it seems like an overly aggressive thing to flag. I think ideally we either:

* limit this warning to when it's a concrete (non-parameterized) type shifted by a literal (non parameter, perhaps context-independent `const` is also ok) amount
* give some pragma for opting an expression out

the latter means that routines aren't truly generic without source modification, so I think that's both less desirable and more work, so I'd like to restrict the warning to the former.