Auto merge of #87794 - bonega:enum_niche_prefer_zero, r=nagisa

Enum should prefer discriminant zero for niche

Given an enum with unassigned zero-discriminant, rust should prefer it for niche selection.
Zero as discriminant for `Option<Enum>` makes it possible for LLVM to optimize resulting asm.

- Eliminate branch when expected value coincides.
- Use smaller instruction `test eax, eax` instead of `cmp eax, ?`
- Possible interaction with zeroed memory?

Example:
```rust

pub enum Size {
    One = 1,
    Two = 2,
    Three = 3,
}

pub fn handle(x: Option<Size>) -> u8 {
    match x {
        None => {0}
        Some(size) => {size as u8}
    }
}
```
In this case discriminant zero is available as a niche.

Above example on nightly:
```asm
 mov     eax, edi
 cmp     al, 4
 jne     .LBB0_2
 xor     eax, eax
.LBB0_2:
 ret
```

PR:
```asm
 mov     eax, edi
 ret
```

I created this PR because I had a performance regression when I tried to use an enum to represent legal grapheme byte-length for utf8.

Using an enum instead of `NonZeroU8` [here](d683304f5d/src/internal/decoder_incomplete.rs (L90))
resulted in a performance regression of about 5%.
I consider this to be a somewhat realistic benchmark.

Thanks to `@ogoffart` for pointing me in the right direction!

Edit: Updated description

src/test/ui/enum-discriminant/niche-prefer-zero.rs

@@ -0,0 +1,14 @@
+// Check that niche selection prefers zero.
+// See https://github.com/rust-lang/rust/pull/87794
+// run-pass
+#[repr(u8)]
+pub enum Size {
+    One = 1,
+    Two = 2,
+    Three = 3,
+}
+
+fn main() {
+    // check that `None` is zero
+    assert_eq!(0, unsafe { std::mem::transmute::<Option<Size>, u8>(None) });
+}