Integer and float modulus are separated into Mod and IMod operator types
to make the code clearer and to make GetOperatorString return the correct
value for both.

Change-Id: Ibfbca2c558bf919b0eab6404d7349f87fe47a18c
Reviewed-on: https://chromium-review.googlesource.com/249692Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>