*re-land with fix for unused var in release*

We checked this at link time in the D3D back-end, but this
restriction applies to all shaders.

TEST=dEQP-GLES2.functional.shaders.fragdata.*
BUG=angleproject:995
BUG=478572

Change-Id: I63258f4de47e658812822f31601cc235f48c0826
Reviewed-on: https://chromium-review.googlesource.com/271470Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>