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/354590Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>