*re-land with fix for AMD/ES2/OpenGL.*

An off-by-one bug slipped in that broke support for these edge case
shaders.

Bug introduced in https://chromium-review.googlesource.com/#/c/266928/

BUG=angleproject:1045
BUG=500116

Change-Id: If44f809d432221d1e17afc407d49e87e0cb7504c
Reviewed-on: https://chromium-review.googlesource.com/277664Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>