nir: Introduce nir_intrinsic_discard_if.

nir: Introduce nir_intrinsic_discard_if.

This is a conditional discard, which takes a boolean source.

Note that we don't generate ir_discard::condition today, so this
shouldn't break drivers (since none implement this intrinsic yet).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>