librustc: Don't ICE when operator traits are not implemented properly.

From my comment on #11450:

The reason for the ICE is because for operators `rustc` does a little bit of magic. Notice that while you implement the `Mul` trait for some type `&T` (i.e a reference to some T), you can simply do `Vec2 {..} * 2.0f32`. That is, `2.0f32` is `f32` and not `&f32`. This works because `rustc` will automatically take a reference. So what's happening is that with `foo * T`, the compiler is expecting the `mul` method to take some `&U` and then it can compare to make sure `T == U` (or more specifically that `T` coerces to `U`). But in this case, the argument of the `mul` method is not a reference and hence the "no ref" error.

I don't think we should ICE in this case since we do catch the mismatched trait/impl method and hence provide a better error message that way.

Fixes #11450