LLVM doesn't know the result of a usize::saturating_add is >= the first argument

[saethlin]

https://godbolt.org/z/67q79vnEW

This function should be optimized away completely, but it isn't:

pub fn demo(x: usize, y: usize) {
    let res = x.saturating_add(y);
    assert!(res >= x);
}

Original example was this, which currently has two bounds checks (that the range end is >= than the start, and that the end of the range is in-bounds), but it should only have one:

pub fn subslice(slice: &[u8], x: usize) {
    let _ = &slice[x..x.saturating_add(10)];
}

[saethlin]

Migrated upstream llvm/llvm-project#63381, @scottmcm says this would be unwise to solve on the rustc side.