ir3: Stop creating dummy dest registers

ir3: Stop creating dummy dest registers

These were a holdover from before the src/dst split and are no longer
necessary. Just don't create any dest registers for instructions that
never have a destination.

This has the side-effect that it becomes easier to replace uses of
dest_regs() with a per-register thing, once we start adding support for
multiple destinations.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11565>