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75dc99b996
fix unsoundness in Step::forward_unchecked for signed integers Fixes #122420 ```rust pub fn foo(a: i8, b: u8) -> i8 { unsafe { a.checked_add_unsigned(b).unwrap_unchecked() } } ``` still compiles down to a single arithmetic instruction ([godbolt](https://rust.godbolt.org/z/qsd3xYWfE)). But we may be losing some loop optimizations if llvm can no longer easily derive that it's a finite counted loop from the no-wrapping flags.