Significantly worse codegen for SIMD shuffles after Rust 1.73.0

See https://godbolt.org/z/jKczKPMMe.

The code is the following:

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

#[no_mangle]
pub unsafe fn custom_shuffle(x: __m128i) -> __m128i {
    let mut tmp1 = _mm_unpacklo_epi8(x, _mm_setzero_si128());
    tmp1 = _mm_shuffle_epi32::<78>(tmp1);
    tmp1 = _mm_shufflelo_epi16::<27>(tmp1);
    tmp1 = _mm_shufflehi_epi16::<27>(tmp1);

    let mut tmp2 = _mm_unpackhi_epi8(x, _mm_setzero_si128());
    tmp2 = _mm_shuffle_epi32::<78>(tmp2);
    tmp2 = _mm_shufflelo_epi16::<27>(tmp2);
    tmp2 = _mm_shufflehi_epi16::<27>(tmp2);

    _mm_packus_epi16(tmp2, tmp1)
}

The regression happened between Rust 1.72.0 and Rust 1.73.0.

Start with the linked Godbolt example and compare the generated code from Rust 1.72.0 and 1.73.0 for the provided SIMD shuffle function. Trace the compiler codegen path responsible for these operations and identify the change that introduced the regression. Done means the example no longer produces significantly worse code than in Rust 1.72.0, with a regression test covering the behavior.