nir/dead_cf: Remove dead control flow after infinite loops.

And after discard-only loops. Otherwise we end up with dead code
which confuses nir_repair_ssa into adding a whole bunch of uses
of undefined. However, for derefs, we sometimes always expect to
get a variable instead of undefined.

Fixes dEQP-VK.graphicsfuzz.write-red-in-loop-nest on radv.

Fixes: c832820c "nir/dead_cf: Repair SSA if the pass makes progress"
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1928


Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
(cherry picked from commit 6da3bf26)