nir/ttn: Silence some warnings at -Og

nir/ttn: Silence some warnings at -Og

   ../src/gallium/auxiliary/nir/tgsi_to_nir.c: In function ‘ttn_mem’:
   ../src/gallium/auxiliary/nir/tgsi_to_nir.c:1724:15: warning: ‘op’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    1724 |       instr = nir_intrinsic_instr_create(b->shader, op);
   ../src/gallium/auxiliary/nir/tgsi_to_nir.c:1697:15: warning: ‘op’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    1697 |       instr = nir_intrinsic_instr_create(b->shader, op);

Add some more unreachable() to make this go away.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>