Integer operations that overflow are defined to wrap in the ESSL
3.00.6 spec. Constant folding that happens inside the shader
translator should also follow the wrapping rules.

The new implementations of wrapping integer addition and subtraction
use unsigned integers to perform calculations. Unsigned integers are
defined to implement arithmetic in modulo 2^n in the C++ spec. This
behavior is also leveraged to implement wrapping unsigned integer
multiplication.

The implementation of wrapping signed integer multiplication is
slightly trickier. The operands are casted to a wider type to perform
the multiplication in a way that doesn't overflow, and then the result
is truncated and casted back to the narrower integer type.

Incorrect tests that expected errors to be generated from integer
overflow in constant folding are removed.

BUG=chromium:637050
TEST=angle_unittests

Change-Id: I0de7e25881d254803455fbf22907c192f49d09ff
Reviewed-on: https://chromium-review.googlesource.com/390252
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>