Iterating all `char`s blocks dead code elimination

This following code does nothing, however does not get optimized away.

pub fn dead_code_elim_failure() {
    for _ in '\0'..'\u{10ffff}' {}
}

example::dead_code_elim_failure::h8fb80fbacc62758b:
        xor     ecx, ecx
        mov     eax, 57344
.LBB0_1:
        cmp     ecx, 55295
        lea     edx, [rcx + 1]
        cmove   edx, eax
        mov     ecx, edx
        cmp     edx, 1114111
        jb      .LBB0_1
        ret

Meta

Tested on:

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: x86_64-unknown-linux-gnu
release: 1.77.2
LLVM version: 17.0.6

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

Start by compiling the reported Rust reproducer with optimization enabled and compare the generated assembly against the issue. Trace the compiler optimization path responsible for the loop and verify that the empty iteration is eliminated without changing observable behavior.