tgsi/exec: implement restartable machine.

tgsi/exec: implement restartable machine.

This lets us restart the machine at a PC value, and exits
the machine when we hit a barrier.

Compute shaders will then execute all the threads up to the
barrier, then restart the machines after the barrier once
all are done.

v2: comment the code a bit, change return types.

Acked-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>