Make opsem for all enums consistent, independent of number of variants and `#[non_exhaustive]`

The way we generate MIR currently means that enum opsem distinguished two classes of enums

• Those with more than 1 variant or a #[non_exhaustive] attribute
• Those with 1 or 0 variants and no #[non_exhaustive]

For the latter, we sometimes omit reading the discriminant since we "already know the result". Also see https://github.com/rust-lang/rust/pull/150681 where this is point 1.

I think we should treat all enums consistently. This example should be considered UB. In particular, I think #[non_exhaustive] should not affect the opsem.

This change was separated out of https://github.com/rust-lang/rust/pull/150681 at the request of @traviscross, so that it can be discussed separately. So, let's discuss. :) @scottmcm and @nikomatsakis already expressed agreement with this next step in that PR.

Cc @meithecatte -- IIRC this had a non-zero crater fallout; where are the results of that again?

Start by reviewing the linked Rust PR #150681, the playground example, and the MIR opsem discussion, including the referenced crater fallout. A complete contribution requires a settled decision on consistent enum discriminant semantics, followed by implementation and validation that the example is treated as UB without special handling for variant count or #[non_exhaustive].