Don't lose GIs when adding column to root table

The TableChangeValidatorState tracked affected and dropped GIs but not unaffected ones. Additionally, root table changes have a stub group in AISMerge and it was assumed that GIs were hooked up later. At some point that no longer became true, probably in one of the refactor/clean-ups. 

Once again this shows how it would be much less fragile if GIs were not special in the ALTER processing (and if there were less moving pieces in the transition from statement to action). But that's a big change so change as little as possible, again.