There are a couple cases where a single shader might happen:

- compute shaders
  (only one stage, no inputs and outputs; separable shouldn't matter)
- vertex shaders with transform feedback
  (we want to retain outputs, but transform feedback varyings are
   specified via the API, not the shader - setting SSO fixes this)
- old shader_test files captured before we started adding "SSO ENABLED".

In any case, it seems harmless or beneficial to enable SSO for all
.shader_test files containing a single shader.

Based on a patch by Marek.

v2: Ignore VP/FP shaders.

Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Timothy Arceri <timothy.arceri@collabora.com>