nv50/ir: handle S8/S16 integers converting to S64

We can't convert directly from signed integers smaller 64 bit to signed
64 bit integers. For 32 bit integers this is handled with SHR and MERGE.
In order to also support 8/16 bit singed integers convert them to 32 bit
first.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18109>