Rollup merge of #122461 - the8472:fix-step-forward-unchecked, r=Amanieu

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.

src/tools/miri/tests/pass/shims/time-with-isolation2.stdout

@@ -1 +1 @@
-The loop took around 7s
+The loop took around 12s