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/278240Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>