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: Iddfe0c21bef3c3a61017ba56d9e1fc001c0a3703
Reviewed-on: https://chromium-review.googlesource.com/277476Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>