Add utility modules for byte extract and update metric reporting

Provides the utility modules required to report byte extract
and byte update metrics. This includes helper functions to
check if given expr is a byte extract or update and an SSA_stept
class member function that returns the relevant ssa expression
from a given SSA step. If the SSA step is a shared_read or
shared_write, return ssa_lhs, else return cond_expr.

Also, added an invariant in the constructor for byte_extract_exprt
and byte_update_exprt to confirm _id is valid for additional
correctness checks.

Author: Natasha Yogananda Jeppu

src/goto-symex/ssa_step.h

@@ -148,6 +148,11 @@ class SSA_stept
   exprt cond_handle;
   std::string comment;
 
+  exprt get_ssa_expr() const
+  {
+    return ((is_shared_read() || is_shared_write()) ? ssa_lhs : cond_expr);
+  }
+
   // for INPUT/OUTPUT
   irep_idt format_string, io_id;
   bool formatted = false;

src/util/byte_operators.h

@@ -25,7 +25,7 @@ Author: Daniel Kroening, kroening@kroening.com
 /// The object can either be interpreted in big endian or little endian, which
 /// is reflected by the \c id of the expression which is either
 /// \c ID_byte_extract_big_endian or \c ID_byte_extract_little_endian
-class byte_extract_exprt:public binary_exprt
+class byte_extract_exprt : public binary_exprt
 {
 public:
   byte_extract_exprt(
@@ -35,6 +35,9 @@ class byte_extract_exprt:public binary_exprt
     const typet &_type)
     : binary_exprt(_op, _id, _offset, _type)
   {
+    INVARIANT(
+      _id == ID_byte_extract_little_endian || _id == ID_byte_extract_big_endian,
+      "byte_extract_exprt: Invalid ID");
   }
 
   exprt &op() { return op0(); }
@@ -84,6 +87,9 @@ class byte_update_exprt : public ternary_exprt
     const exprt &_value)
     : ternary_exprt(_id, _op, _offset, _value, _op.type())
   {
+    INVARIANT(
+      _id == ID_byte_update_little_endian || _id == ID_byte_update_big_endian,
+      "byte_update_exprt: Invalid ID");
   }
 
   DEPRECATED(SINCE(2019, 5, 21, "use set_op or as_const instead"))
@@ -111,6 +117,13 @@ class byte_update_exprt : public ternary_exprt
   const exprt &value() const { return op2(); }
 };
 
+template <>
+inline bool can_cast_expr<byte_update_exprt>(const exprt &base)
+{
+  return base.id() == ID_byte_update_little_endian ||
+         base.id() == ID_byte_update_big_endian;
+}
+
 inline const byte_update_exprt &to_byte_update_expr(const exprt &expr)
 {
   PRECONDITION(expr.operands().size() == 3);