globals in DS graph


Every graph has a separate copy of the node for a global.
There is no sharing of nodes between graphs. For any particular
global object (a GlobalValue*), you can find the node for that
global in each graph from the ScalarMap for that graph.

Eventually, irrelevant^(*) globals within each function's graph will
be deleted but other globals will stay the same as they are now.
When that happens, we will need to fully implement the globals
graph for our graph construction algorithm, but even then many
clients (including yours) may never really need to know about
the globals graph.

(*) A global is irrelevant in a function if it is not reachable from
    and cannot reach any node that is accessible within that function.
    Such a global cannot be used to reference or modify any accessible
    node within that function.