UB in constant exprs

We need a story for UB in constant exprs. Do the underlying APIs provide deterministic behavior? Should the generated code be made deterministic or the UB exposed in the DSL?

E.g.
```
%a = shl nsw i8 %x, C1
%b = icmp sgt %a, C0
  =>
%b = icmp sgt %x, (C0 >> C1)
```

What if `C1 >= width(C1)` in `C0 >>C1`?