Use code_declt::set_initial_value

To avoid directly writing to the operands of a `code_declt`.

Author: thomasspriggs

src/goto-instrument/dump_c.cpp

@@ -654,7 +654,7 @@ void dump_ct::cleanup_decl(
 
   if(value)
   {
-    decl.operands().resize(1);
+    decl.set_initial_value({});
   }
 
   goto_programt tmp;

src/goto-instrument/goto_program2code.cpp

@@ -477,7 +477,7 @@ goto_programt::const_targett goto_program2codet::convert_decl(
     {
       if(next->is_assign())
       {
-        d.copy_to_operands(next->get_assign().rhs());
+        d.set_initial_value({next->get_assign().rhs()});
       }
       else
       {