[imcc] fix -O2 empty ins->next in used_once [GH #1042]

If the deleted op leads to the last op, which has no op->next
break the used_once loop.

Fixes GH #1042 t/op/exit_6.pasm
    set I0, 0 # used_once => deleted
    end
ChangeLog: Add -O1 and -O2 to fulltest