Methods for ai_baset to allow access to the ai_base_domaint for a location.

Previously, to access the domain for at a particular location, you had to
use the operator[] interface in ait<domainT> (or subclass), which meant that
the code had to be written / templated / duplicated for each domainT, even
when the only interface that was being used was the ai_base_domaint one.
This is the case for various tasks in goto-analyzer.  This interface should
also support flow-insensitive or context-dependent AI engines.

Author: martin

src/analyses/ai.h

@@ -162,6 +162,17 @@ class ai_baset
     fixedpoint(goto_function.body, goto_functions, ns);
   }
 
+  /// Returns the abstract state before the given instruction
+  virtual const ai_domain_baset & abstract_state_before(
+    goto_programt::const_targett t) const = 0;
+
+  /// Returns the abstract state after the given instruction
+  virtual const ai_domain_baset & abstract_state_after(
+    goto_programt::const_targett t) const
+  {
+    return abstract_state_before(std::next(t));
+  }
+
   virtual void clear()
   {
   }
@@ -370,6 +381,12 @@ class ait:public ai_baset
     return it->second;
   }
 
+  const ai_domain_baset & abstract_state_before(
+    goto_programt::const_targett t) const override
+  {
+    return (*this)[t];
+  }
+
   void clear() override
   {
     state_map.clear();