pan/va: Terminate helper threads

pan/va: Terminate helper threads

On Bifrost, to terminate helper threads we set the td bit on the clause. On
Valhall, we need to use the .discard flow control. Extend the flow control NOP
insertion to insert NOP.discard where necessary to terminate helper threads.
This should reduce wasted work in fragment shaders.

This requires fairly involved data flow analysis, but the handling here should
be optimal.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16804>