Performance regression: rustc failed to optimize specific x86-64 SIMD intrinsics after 1.75.0

Code

I tried this code:

https://rust.godbolt.org/z/KG4cT6aPK

use std::arch::x86_64::*;

#[target_feature(enable = "avx2")]
pub unsafe fn decode(
    x: __m256i,
    ch: __m256i,
    ct: __m256i,
    dh: __m256i,
    dt: __m256i,
) -> Result<__m256i, ()> {
    let shr3 = _mm256_srli_epi32::<3>(x);

    let h1 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(ch, x));
    let h2 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(dh, x));

    let o1 = _mm256_shuffle_epi8(ct, h1);
    let o2 = _mm256_shuffle_epi8(dt, h2);

    let c1 = _mm256_adds_epi8(x, o1);
    let c2 = _mm256_add_epi8(x, o2);

    if _mm256_movemask_epi8(c1) != 0 {
        return Err(());
    }

    Ok(c2)
}

I expected to see this happen: This code should emit two vpavgb instructions.

Instead, this happened: One of the vpavgb instructions is missing.

https://github.com/Nugine/simd/issues/43

Version it worked on

It most recently worked on: 1.74.1

Version with regression

1.75.0 ~ nightly

rustc 1.79.0-nightly (dbce3b43b 2024-04-20)
binary: rustc
commit-hash: dbce3b43b6cb34dd3ba12c3ec6f708fe68e9c3df
commit-date: 2024-04-20
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

Start with the linked rust.godbolt.org example and compare rustc 1.74.1 with 1.75.0 or the referenced nightly, checking the generated x86-64 assembly for both vpavgb instructions. Trace the compiler or LLVM change responsible and add a regression test; done means both instructions are emitted for the provided SIMD code.