negateS -> negateI

The same negate should work fine for unsigned, right?
This seems to eliminate the need for kInvaid.

    ~ $ cat test.c
    int main(void) {
        unsigned u = 0;
        do {
            unsigned nu = -u,
                     ns = -(int)u;
            if (nu != ns) {
                return 1;
            }
        } while (++u != 0);

        return 0;
    }
    ~ $ clang -Os test.c
    ~ $ ./a.out && echo ok
    ok

Change-Id: If4eec8aa24dce634e3196c8631adec4362a3b69b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215045
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed