gallivm/nir: extract a valid texture index according to exec_mask.
When using indirect textures, some lanes may not be active, particularly in a loop, so as with some other areas, extracting the first correct lane is needed here.
KHR-GL45.texture_barrier.* on zink.
Fixes: e168d148 ("gallivm/nir: handle non-uniform texture offsets")
Edited by Dave Airlie