nir: remove simple dead if detection from nir_opt_dead_cf()

nir: remove simple dead if detection from nir_opt_dead_cf()

This was probably useful when it was first written, however it
looks to be no longer necessary.

As far as I can tell these days dce is smart enough to remove useless
instructions from if branches. Once this is done
nir_opt_peephole_select() will end up removing the empty if.

Removing this support reduces the dolphin uber shader compilation
time spent in nir_opt_dead_cf() by a little over 7x.

No shader-db changes on i965 or radeonsi.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>