nir: Fix the Mesa build without -DDEBUG.

nir: Fix the Mesa build without -DDEBUG.

With -DDEBUG -UNDEBUG, this assert uses reg_state::stack_size, which
doesn't exist, breaking the build:

assert(state->states[index].index < state->states[index].stack_size);

Switch it to ifndef NDEBUG, so the field will exist if the assertion
actually generates code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>