Use s.empty() or !s.empty() instead of comparison to irep_idt()

This avoids constructing an object when we can just use an integer comparison.

Author: tautschnig

jbmc/src/java_bytecode/java_entry_point.cpp

@@ -547,7 +547,7 @@ main_function_resultt get_main_symbol(
     irep_idt main_symbol_id=
       resolve_friendly_method_name(config.main, symbol_table, error_message);
 
-    if(main_symbol_id==irep_idt())
+    if(main_symbol_id.empty())
     {
       message.error()
         << "main symbol resolution failed: " << error_message << messaget::eom;

jbmc/src/java_bytecode/java_object_factory.cpp

@@ -329,7 +329,7 @@ class recursion_set_entryt
   /// Removes erase_entry (if set) from the controlled set.
   ~recursion_set_entryt()
   {
-    if(erase_entry!=irep_idt())
+    if(!erase_entry.empty())
       recursion_set.erase(erase_entry);
   }
 
@@ -342,10 +342,8 @@ class recursion_set_entryt
   /// \return true if added to the set (and therefore owned by this object)
   bool insert_entry(const irep_idt &entry)
   {
-    INVARIANT(
-      erase_entry==irep_idt(),
-      "insert_entry should only be called once");
-    INVARIANT(entry!=irep_idt(), "entry should be a struct tag");
+    INVARIANT(erase_entry.empty(), "insert_entry should only be called once");
+    INVARIANT(!entry.empty(), "entry should be a struct tag");
     bool ret=recursion_set.insert(entry).second;
     if(ret)
     {

src/ansi-c/parser_static.inc

@@ -224,7 +224,7 @@ static void make_subtype(typet &dest, typet &src)
       }
       else if(p->is_nil())
         assert(false);
-      else if(p->id()==irep_idt())
+      else if(p->id().empty())
         assert(false);
       else
       {
@@ -247,7 +247,7 @@ static void make_subtype(typet &dest, typet &src)
             auto &merged_type = to_merged_type(*p);
             p=&merged_type.last_type();
           }
-          else if(p->id()==irep_idt())
+          else if(p->id().empty())
             assert(false);
           else if(p->is_nil())
             assert(false);

src/goto-programs/instrument_preconditions.cpp

@@ -75,8 +75,7 @@ replace_symbolt actuals_replace_map(
   std::size_t count=0;
   for(const auto &p : parameters)
   {
-    if(p.get_identifier()!=irep_idt() &&
-       arguments.size()>count)
+    if(!p.get_identifier().empty() && arguments.size() > count)
     {
       const exprt a =
         typecast_exprt::conditional_cast(arguments[count], p.type());

src/solvers/smt2/smt2_conv.cpp

@@ -234,7 +234,7 @@ constant_exprt smt2_convt::parse_literal(
 
   mp_integer value;
 
-  if(src.id()!=irep_idt())
+  if(!src.id().empty())
   {
     const std::string &s=src.id_string();