Warn user when the whole function is optimized to a `ud2`

Code

pub fn bad_func1() -> i32 {
    let arr = [1, 2, 3];
    unsafe { *arr.get_unchecked(5) }
}

Current output

No warning, but the assembly only contains a single `ud2` instruction, which will terminate the program with `SIGILL` if executed.

Desired output

Warn user that this function has undefined behavior and will not work as intended.

Rationale and extra context

This warning should be emitted at very last stages of compiling, and it might be very hard to implement in the current compiler architecture. But it can save time of the developer, specially when there is no test coverage for that function.

It should be a warning and not a hard error, since optimizations can work differently from version to version.

Other cases

Rust Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

Anything else?

No response

Use the provided Rust reproducer with rustc 1.86.0 as the starting case, then investigate the compiler's final compilation stages where the whole function becomes a single ud2. Done means emitting a warning rather than a hard error for this case while leaving optimization behavior intact.