ir3: optimize read_first.macro to a mov

Moves to shared registers keep the value written by the lowest active
fiber. This means that read_first.macro can simply be turned into a mov
and doesn't need a getone block.

Signed-off-by: Job Noorman <jnoorman@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26950>