Auto merge of #122459 - Nadrieril:sort-eq, r=oli-obk

match lowering: sort `Eq` candidates in the failure case too

This is a slight tweak to MIR gen of matches. Take a match like:
```rust
match (s, flag) {
    ("a", _) if foo() => 1,
    ("b", true) => 2,
    ("a", false) => 3,
    (_, true) => 4,
    _ => 5,
}
```
If we switch on `s == "a"`, the first candidate matches, and we learn almost nothing about the second candidate. So there's a choice:
1. (what we do today) stop sorting candidates, keep the "b" case grouped with everything below. This could allow us to be clever here and test on `flag == true` next.
2. (what this PR does) sort "b" into the failure case. The "b" will be alone (fewer opportunities for picking a good test), but that means the two "a" cases require a single test.

Today, we aren't clever in which tests we pick, so this is an unambiguous win. In a future where we pick tests better, idk. Grouping tests as much as possible feels like a generally good strategy.

This was proposed in https://github.com/rust-lang/rust/issues/29623 (9 years ago :D)

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