Constant folding % used to generate results from division instead of
results from modulus. Fix this and introduce a test for this and other
integer constant folding, that checks that the right constant values are
found in the IntermNode tree.

BUG=angle:916
TEST=angle_unittests

Change-Id: I75e59b7734eb206378a1b2957af0a5437a90097c
Reviewed-on: https://chromium-review.googlesource.com/249691Reviewed-by: Gregoire Payen de La Garanderie <Gregory.Payen@imgtec.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Nicolas Capens <capn@chromium.org>