module BackwardsDataFlow: 
val compute : Cil.stmt list -> unit
Fill in the T.stmtStartData, given a number of initial statements to 
 start from (the sinks for the backwards data flow). All of the statements
 (not just the initial ones!) must have some entry in T.stmtStartData 
 If you want to use bottom for the initial data, you should pass the
 complete list of statements to compute, so that everything is visited.
 find_stmts may be useful here.