Currently, the nvptx backend is only neutering the worker axis when
propagating variables used in conditional expressions across the worker
and vector axes. That's a problem with the worker-state spill and fill
propagation implementation because all of the vector threads in worker 0
all write the the same address location being spilled. As the attached
test case demonstrates, this might cause an infinite loop depending on
the values in the vector threads being propagated.
This patch fixes this issue by introducing a new worker-vector
predicate, so that both the worker and vector threads can be predicated
together, not separately. I.e., instead of first neutering worker axis,
then neutering the vector axis, this patch uses a single predicate for
tid.x == 0 && tid.y == 0.
Is this patch ok for trunk?