For transform feedback, any vertex shader output can be read from
and which outputs will be read by transform feedback buffers is not
known at compile time. For that reason, any output being written to
in the vertex shader code shouldn't be optimized out.

Change-Id: Ia4322c43b7e3308ec5d930650e70aacf032dc6ec
Reviewed-on: https://swiftshader-review.googlesource.com/5051Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>