Pass specific annotation data for trees

[Daniel Berlin]

If doing this, you have to be careful that a pass isn't called from
within another pass. out-of-ssa can be called from within DOM, for
instance, so dom and out-of-ssa can't have their pass info in a union.
Yes they can, they just have to have a substructure that is shared 
between them that is the first member, and part of the union :)

This is the same situation we have with tree_nodes and all the
bits/flags found therein.
yup yup

Andrew