Patch to allow Ada to work with tree-ssa

[Richard Kenner]

    Surely this walk is only applicable to some particular traversal of the
    tree, and thus a bit is not needed at all.  Surely the one walk that
    needs it can explicitly choose to iterate the fields simply by
    recursively calling walk_tree.

That *might* be right, but I'm not convinced of that.

    Of course, he's not given any specifics whatsoever about what led him
    to modify walk_tree.  Only that at some nebulous point he needed to see
    the fields.

The motivation was sharing: TYPE_SIZE can't be shared with an expression
in DECL_FIELD_OFFSET, for example, or gimplification has problems.