Don't unpack un-referenced varyings in the pixel shader. Transform feedback varyings that were optimized out were still being unpacked causing HLSL compilation failures. This was triggering failures in the conformance2/state/gl-object-get-calls.html test. BUG=angleproject:1422 Change-Id: I297cccd5b99435dfb69a3c2b0fd3086b6ddf0b3a Reviewed-on: https://chromium-review.googlesource.com/354590 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>