The optimizers need to associate attributes with statements and
variables during the optimization process. For instance, we need to
know what basic block a statement belongs to or whether a variable
has aliases. All these attributes are stored in data structures
called annotations which are then linked to the field ann in
struct tree_common.
Presently, we define annotations for statements (stmt_ann_t),
variables (var_ann_t) and SSA names (ssa_name_ann_t).
Annotations are defined and documented in tree-flow.h.