st/nine: remove some shader unused code

Signed-off-by: Axel Davy <axel.davy@ens.fr>
st/nine: better logic for loop

Signed-off-by: Axel Davy <axel.davy@ens.fr>
st/nine: Match REP implementation to LOOP

Signed-off-by: Axel Davy <axel.davy@ens.fr>
st/nine: use USGE instead of USEQ for loops

It is more safe

Signed-off-by: Axel Davy <axel.davy@ens.fr>