It's a bit weird that free_dom_edge_info leaves a dangling pointer in 
e->aux.  Not sure what I was thinking.


There's two callers.  One wipes e->aux immediately after the call, the 
other attaches a newly created object immediately after the call.  So we 
can wipe e->aux within the call and simplify one of the two call sites.

This is preparatory work for a minor optimization where we want to 
detect another class of edge equivalences in DOM (until something better 
is available) and either attach them an existing edge_info structure or 
create a new one if one doesn't currently exist for a given edge.

Bootstrapped and regression tested on x86_64.  Installing on the trunk.


Jeff