[llvm] r257697 - Convert a few assert failures into proper errors.

[Rafael Espíndola via llvm-commits]

> I was talking with a few folks about doing this before the holidays, but
> there's still quite a bit of churn in WebAssembly so we were worried of
> going out of experimental too early. Maybe now is a good time to do so? I
> can start a thread on the mailing list if you think so.

Yes, I think it should be fine.

Thanks,
Rafael