High bit dimension warning is brittle.

You are supposed to get a warning if you use a value with bit 31 set as a dimension. However, currently this will only trigger where array type unification is actually required and cannot be short-circuited.

Fuzz test domain specs, for example, are currently "exempt" from the warning due to happenstance (unification gets short circuited), but any change that starts to force unification would make fuzz domains like `u32:0..0xffffffff` trigger this warning. It's unclear if that would then be desired or a bug. Such domains can lower and work perfectly fine.

We need to shore up this warning and make it more guaranteed if we really want it.