aco: decrease the uses of other copy operations after splitting/removing

aco: decrease the uses of other copy operations after splitting/removing

For copies like v[7:8] = v[8:9], what currently happens is:
- do_copy() will skip the second dword
- the uses of the second dword will be reduced to 0
- the copy operation will be removed from the map
and v8 will never be set to v9.

So just decrease the uses of other operations after splitting or removing
the current operation, so: "v8 = v9" will be split off, it's uses reduced
and then the new copy will be done in the next iteration.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4686>