`opt-level=s` doesn't enable the SLP vectorizer (fails to use packed floats)

For reference: https://godbolt.org/z/dv6rEeYWe

Compile the following code under opt-level=s:

pub fn subps(a: &[f32; 4], b: &[f32; 4], c: &mut [f32; 4]) {
    let c0 = a[0] - b[0];
    let c1 = a[1] - b[1];
    let c2 = a[2] - b[2];
    let c3 = a[3] - b[3];
    c[0] = c0;
    c[1] = c1;
    c[2] = c2;
    c[3] = c3;
}

I expected:

subps:
        movups  xmm0, xmmword ptr [rdi]
        movups  xmm1, xmmword ptr [rsi]
        subps   xmm0, xmm1
        movups  xmmword ptr [rdx], xmm0
        ret

Instead, I got:

subps:
        movss   xmm0, dword ptr [rdi]
        movss   xmm1, dword ptr [rdi + 4]
        subss   xmm0, dword ptr [rsi]
        subss   xmm1, dword ptr [rsi + 4]
        movss   xmm2, dword ptr [rdi + 8]
        subss   xmm2, dword ptr [rsi + 8]
        movss   xmm3, dword ptr [rdi + 12]
        subss   xmm3, dword ptr [rsi + 12]
        movss   dword ptr [rdx], xmm0
        movss   dword ptr [rdx + 4], xmm1
        movss   dword ptr [rdx + 8], xmm2
        movss   dword ptr [rdx + 12], xmm3
        ret

This is roughly similar to clang's -O1 level. Since opt-level=s should optimize for size, the inability to use packed floats is an issue.

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rustc --version --verbose:

rustc 1.90.0-nightly (11ad40bb8 2025-06-28)
binary: rustc
commit-hash: 11ad40bb839ca16f74784b4ab72596ad85587298
commit-date: 2025-06-28
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7
Compiler returned: 0

Start with the minimal Rust reproducer and compare its opt-level=s assembly with the expected packed-float instructions shown in the issue and on Godbolt. Investigate how rustc's size-optimization settings configure LLVM's SLP vectorizer. Done means the reproducer uses packed float operations under opt-level=s without regressing the size-oriented behavior.