GLSL function texelfetchoffset will be translated into texture.Load in
ANGLE. In D3D there is a note that When one or more of the coordinates
in Location exceeds the u, v, or w mipmap level dimensions of the
texture, Load returns zero in all components, but in glsl there is no
such restriction, which will cause the WebGL 2 dEQP test
deqp/functional/gles3/shadertexturefunction/texelfetchoffset.html fail
on Windows with Intel GPU.

Adapted from ExpandIntegerPowExpressions.cpp, this patch adds a
translation from texelFetchOffset into texelFetch to work around this
issue.

BUG=angleproject:1469

Change-Id: Iecfb9570472036acf5960789bdb1a63f191316be
Reviewed-on: https://chromium-review.googlesource.com/367883
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>