compiler: use NDEBUG to guard asserts

nir_validate.c's #endif already had the correct NDEBUG comment

Fixes: dcb1acde "nir/validate: Only build in debug mode"
Fixes: 9ff71b64 "i965/nir: Validate that NIR passes call nir_metadata_preserve()"
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 7b85b9b8)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/compiler/nir/nir.h