nir/lower_outputs_to_temporaries: Reparent the output name

nir/lower_outputs_to_temporaries: Reparent the output name

We copy the output, make the old output the temporary, and give the
temporary a new name.  The copy keeps the pointer to the old name.  This
works just fine up until the point where we lower things to SSA and delete
the old variable and, with it, the name.  Instead, we should re-parent to
the copy.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>