i965/vec4: Don't emit MOVs for unused URB slots.

i965/vec4: Don't emit MOVs for unused URB slots.

Otherwise we'd emit a MOV from the null register (which isn't allowed).

Helps 24 programs in shader-db (the geometry shaders in GSCloth):

instructions in affected programs:     302 -> 262 (-13.25%)

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>