Initialize working set to first instruction only

All other instructions, if reachable, will be seen via visit.
Unreachable instructions should have their states remain bottom.

Author: tautschnig

src/analyses/ai.cpp

@@ -225,9 +225,11 @@ bool ai_baset::fixedpoint(
 {
   working_sett working_set;
 
-  // We will put all locations at least once into the working set.
-  forall_goto_program_instructions(i_it, goto_program)
-    put_in_working_set(working_set, i_it);
+  // Put the first location in the working set
+  if(!goto_program.empty())
+    put_in_working_set(
+      working_set,
+      goto_program.instructions.begin());
 
   bool new_data=false;