nir/lower_task_shader: fix task payload corruption when shared memory workaround...

nir/lower_task_shader: fix task payload corruption when shared memory workaround is enabled

We were not taking into account that when all invocations within workgroup
are active, we'll copy more data than needed, corrupting task payload
of other workgroups.

Fixes: 8aff8d3dd42 ("nir: Add common task shader lowering to make the backend's job easier.")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20080>