We would use different code for all of our input and ouput stages
in HLSL (vertex output, GS input, GS output, PS input). This was
causing an HLSL error and some bugs in certain transform feedback
instances.

Fix this by using a helper method to use compatible code for each
of the stages.

BUG=angle:667

Change-Id: Idada969a775eccb5788723e0c7fa3ade9cb2d9ec
Reviewed-on: https://chromium-review.googlesource.com/203778Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>