aco: check if multiplication/clamp is live when applying output modifier

aco: check if multiplication/clamp is live when applying output modifier

It's possible that a multiplication/clamp is dead code and the single use
is from a different user.

Fixes portal rendering in Path of Exile when global illumination is
enabled.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-By: Timur Kristóf <timur.kristof@gmail.com>
Fixes: 93c8ebfa780 ('aco: Initial commit of independent AMD compiler')
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3081>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3081>