aco: validate that SMEM operands can use fixed registers

To fix a validation error when loading the scalar tma buffer
descriptor because it's not a temp but a fixed reg (tma_lo/tma_hi).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6384>