glsl: fix optimization of discard nested multiple levels

glsl: fix optimization of discard nested multiple levels

The order of optimizations can lead to the conditional discard optimization
being applied twice to the same discard statement. In this case, we must
ensure that both conditions are applied.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96762
Cc: mesa-stable@lists.freedesktop.org
Tested-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>