builder: allow unsigned integers in neg and exactsdiv.

[eddyb]

rustc_codegen_ssa relies on signed operations on unsigned integers for e.g. size_of_val.

---

TODO: move this from the compute shader (see diff) to a proper test:

core::mem::size_of_val(
    const {
        struct S<T: ?Sized>(T);
        &S([]) as &S<[()]>
    },
)