Return branches not correctly merged in size-optimized x86-64 assembly

Godbolt link (includes this and another buggy example)

I tried this code:

pub fn parse_u32_digit(acc: u32, byte: u8) -> Option<u32> {
    let max = if acc == 429496729 { 5 } else { 9 };
    if acc <= 429496729 {
        let v = (byte as u32).wrapping_sub(b'0' as u32);
        if v <= max {
            return Some(acc.wrapping_mul(10).wrapping_add(v));
        }
    }
    None
}

I expected to see this happen: assembly that looks something like this:

parse_u32_digit:
        xor     eax, eax
        cmp     edi, 429496729
        ja      .LBB1_1
        setne   cl
        movzx   ecx, cl
        lea     r8d, [4*rcx + 5]
        movzx   ecx, sil
        add     ecx, -48
        cmp     ecx, r8d
        ja      .LBB1_4
        lea     eax, [rdi + 4*rdi]
        lea     edx, [rcx + 2*rax]
        mov     eax, 1
.LBB1_1:
.LBB1_4:
        ret

Instead, this happened: the following assembly:

parse_u32_digit:
        xor     eax, eax
        cmp     edi, 429496729
        ja      .LBB1_1
        setne   cl
        movzx   ecx, cl
        lea     r8d, [4*rcx + 5]
        movzx   ecx, sil
        add     ecx, -48
        cmp     ecx, r8d
        ja      .LBB1_4
        lea     eax, [rdi + 4*rdi]
        lea     edx, [rcx + 2*rax]
        mov     eax, 1
.LBB1_4:
        ret
.LBB1_1:
        ret

Meta

rustc --version --verbose:

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-unknown-linux-gnu
release: 1.79.0
LLVM version: 18.1.7
Compiler returned: 0

Start with the Rust reproducer in the issue and the linked Godbolt example, using rustc 1.79.0 with -C opt-level=s on x86-64. Compare the generated branch layout with the expected and actual assembly, then trace the relevant compiler optimization path. Done means the return branches are correctly merged without changing the reported behavior.