Fix unreachable on bin op between numeric and undefined

[Vexu]

Closes #4004

[andrewrk]

I wouldn't expect you to find this information, given the fact that we don't have the language specification yet (#75), but have a look at #1947 for the general idea here.

In summary, mathematical operations on undefined operands produce undefined values as results, but not undefined behavior. However, branching on undefined values is undefined behavior. So the expected behavior here is for 5 == undefined to produce the value @as(bool, undefined).

For what it's worth, I'm planning on starting the draft of the language specification soon, within a few weeks here.

I pushed a05150e to fix #4004.