Remove useTemporaryIndex() in function RewriteTexelFetchOffset since
no temporary variables will be created in this function.

BUG=angleproject:1469

Change-Id: Ibed56f0ba5c89b6ae1ab1611de39514c3fbb600f
Reviewed-on: https://chromium-review.googlesource.com/387198Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>