aco/spill: reload spilled exec masks directly to exec

aco/spill: reload spilled exec masks directly to exec

This handles the case of
   exec = p_linear_phi %a, %b
where %a or %b might have been spilled.
By directly reloading these variables into the exec mask register,
we can avoid additional CSSA parallelcopies.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9196>