freedreno/ir3: don't DCE ij_pix if used for pre-fs-texture-fetch

freedreno/ir3: don't DCE ij_pix if used for pre-fs-texture-fetch

When we enable pre-dispatch texture fetch, we could have a scenario
where the barycentric i/j coord sysval is not used in the shader, but
only used for the varying fetch for the pre-dispatch texture fetch.
In this case we need to take care not to DCE this sysval.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>