Avoiding Range Checks in Slice Expressions

["Nordlöw"]

> If they are range-checked, it would be a good addition to the 
> optimizer to remove them. My guess is that it is range-checked.
>

$/n is of course always within range if n is positive integer >= 
1.

But what about in the general indexing/slicing case? In that case 
it would be useful if we could reuse value range propagation 
(VRP) in DMD, to figure out which other expressions that don't 
need range checking.