In this particular case, we just did a (lhs)op(rhs), ignoring the case
that lhs and rhs might have different signedness.
This CL changes that to use the proper Cmp##op##Impl implementation,
which does two comparisions for signed-vs-unsigned checks, avoiding
compiler errors.

R=ishell@chromium.org

Review-Url: https://codereview.chromium.org/2642383002
Cr-Commit-Position: refs/heads/master@{#42566}