Avoid uneccessary calls to guardt::as_expr()

We aim at providing other implementations of guardt which may not be
encoded using exprt only.
In that case, `as_expr` may not be a trivial operation as it is now, so
it is better to minimize the number of times it gets called.

Author: romainbrenguier

src/analyses/goto_check.cpp

@@ -1407,16 +1407,15 @@ void goto_checkt::add_guarded_claim(
     return;
 
   // add the guard
-  exprt guard_expr=guard.as_expr();
 
   exprt new_expr;
 
-  if(guard_expr.is_true())
+  if(guard.is_true())
     new_expr.swap(expr);
   else
   {
     new_expr=exprt(ID_implies, bool_typet());
-    new_expr.move_to_operands(guard_expr, expr);
+    new_expr.add_to_operands(guard.as_expr(), std::move(expr));
   }
 
   if(assertions.insert(new_expr).second)

src/goto-symex/goto_symex_state.cpp

@@ -384,6 +384,7 @@ bool goto_symex_statet::l2_thread_read_encoding(
   ssa_exprt ssa_l1=expr;
   ssa_l1.remove_level_2();
   const irep_idt &l1_identifier=ssa_l1.get_identifier();
+  const exprt guard_as_expr = guard.as_expr();
 
   // see whether we are within an atomic section
   if(atomic_section_id!=0)
@@ -430,11 +431,15 @@ bool goto_symex_statet::l2_thread_read_encoding(
       read_guard |= a_s_read_guard;
     }
 
-    exprt cond=read_guard.as_expr();
+    guardt cond = read_guard;
     if(!no_write.op().is_false())
-      cond=or_exprt(no_write.op(), cond);
+    {
+      guardt no_write_guard;
+      no_write_guard.add(no_write.op());
+      cond |= no_write_guard;
+    }
 
-    if_exprt tmp(cond, ssa_l1, ssa_l1);
+    if_exprt tmp(cond.as_expr(), ssa_l1, ssa_l1);
     set_l2_indices(to_ssa_expr(tmp.true_case()), ns);
 
     if(a_s_read.second.empty())
@@ -460,7 +465,7 @@ bool goto_symex_statet::l2_thread_read_encoding(
     record_events=record_events_bak;
 
     symex_target->assignment(
-      guard.as_expr(),
+      guard_as_expr,
       ssa_l1,
       ssa_l1,
       ssa_l1.get_original_expr(),
@@ -498,11 +503,7 @@ bool goto_symex_statet::l2_thread_read_encoding(
   // and record that
   INVARIANT_STRUCTURED(
     symex_target!=nullptr, nullptr_exceptiont, "symex_target is null");
-  symex_target->shared_read(
-    guard.as_expr(),
-    expr,
-    atomic_section_id,
-    source);
+  symex_target->shared_read(guard_as_expr, expr, atomic_section_id, source);
 
   return true;
 }