Single-variant exception for `match` consequences for `#[non_exhaustive]`

bar/src/lib.rs:
```rust
#[non_exhaustive]
#[repr(u8)]
pub enum Thing {
    One(u8) = 0,
}
```

src/main.rs:
```rust
use std::mem::MaybeUninit;

use bar::Thing;

fn main() {
    let buffer: [MaybeUninit<u8>; 2] = [MaybeUninit::uninit(), MaybeUninit::new(0u8)];
    let ptr: *const Thing = (&raw const buffer).cast();
    unsafe {
        match *ptr {
            Thing::One(ref _val) => {}
            _ => {}
        }
    }
}
```

Running the above code in Miri detects UB as follows:
```
error: Undefined Behavior: reading memory at alloc168[0x0..0x1], but memory is uninitialized at [0x0..0x1], and this operation requires initialized memory
 --> src/main.rs:9:15
  |
9 |         match *ptr {
  |               ^^^^ Undefined Behavior occurred here
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
  = note: BACKTRACE:
  = note: inside `main` at src/main.rs:9:15: 9:19

Uninitialized memory occurred at alloc168[0x0..0x1], in this allocation:
alloc168 (stack variable, size: 2, align: 1) {
    __ 00                                           │ ░.
}

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error
```

However, if I delete the `#[non_exhaustive]` line, Miri does not detect any UB.

This seems incorrect. I believe that `#[non_exhaustive]` is supposed to only affect whether code compiles. It should not affect how code behaves.

It seems that, without `#[non_exhaustive]`, rust does not even read the discriminant. But with `#[non_exhaustive]`, rust does.

Related to:
- #142394
- #146803
- #146590
- https://github.com/rust-lang/rust/pull/138961
- https://github.com/rust-lang/reference/pull/1837
- https://github.com/rust-lang/cargo/issues/16417
- https://github.com/rust-lang/rust/pull/150681

### Meta

`rustc --version --verbose`:
```
rustc 1.92.0-nightly (844264add 2025-10-14)
binary: rustc
commit-hash: 844264adda6f41ca6d0d61c4bcac0f263fc5072f
commit-date: 2025-10-14
host: aarch64-apple-darwin
release: 1.92.0-nightly
LLVM version: 21.1.3
```

`cargo miri --version`:
```
miri 0.1.0 (844264adda 2025-10-14)
```

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Single-variant exception for `match` consequences for `#[non_exhaustive]` #147722

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Single-variant exception for `match` consequences for `#[non_exhaustive]` #147722