ir3/spill: Mark root as non-spillable after inserting
We have to mark the root as non-spillable in case the interval is the child of some other interval, but we can't know whether it's the child of some other interval until it's been inserted. Move the setting of cant_spill below the insertion. This prevents us from using a bogus parent value. Fixes: 613eaac7 ("ir3: Initial support for spilling non-shared registers") Part-of: <mesa/mesa!13650> (cherry picked from commit db566904)