Infer hidden field in SSA_assignment_stept

We can deduce it from the assignment type rather than having to give it
to the constructor.

Author: romainbrenguier

src/goto-symex/ssa_step.cpp

@@ -222,8 +222,7 @@ SSA_assignment_stept::SSA_assignment_stept(
   exprt _original_full_lhs,
   exprt _ssa_rhs,
   exprt _cond_expr,
-  symex_targett::assignment_typet _assignment_type,
-  bool _hidden)
+  symex_targett::assignment_typet _assignment_type)
   : SSA_stept(source, goto_trace_stept::typet::ASSIGNMENT)
 {
   guard = std::move(_guard);
@@ -233,5 +232,8 @@ SSA_assignment_stept::SSA_assignment_stept(
   ssa_rhs = std::move(_ssa_rhs);
   assignment_type = _assignment_type;
   cond_expr = std::move(_cond_expr);
-  hidden = _hidden;
+  hidden =
+    assignment_type != symex_targett::assignment_typet::STATE &&
+    assignment_type !=
+    symex_targett::assignment_typet::VISIBLE_ACTUAL_PARAMETER;
 }

src/goto-symex/ssa_step.h

@@ -206,8 +206,7 @@ class SSA_assignment_stept : public SSA_stept
     exprt original_full_lhs,
     exprt ssa_rhs,
     exprt cond_expr,
-    symex_targett::assignment_typet assignment_type,
-    bool hidden);
+    symex_targett::assignment_typet assignment_type);
 };
 
 #endif // CPROVER_GOTO_SYMEX_SSA_STEP_H

src/goto-symex/symex_target_equation.cpp

@@ -122,9 +122,7 @@ void symex_target_equationt::assignment(
     original_full_lhs,
     ssa_rhs,
     equal_exprt(ssa_lhs, ssa_rhs),
-    assignment_type,
-    assignment_type != assignment_typet::STATE &&
-      assignment_type != assignment_typet::VISIBLE_ACTUAL_PARAMETER});
+    assignment_type});
 
   merge_ireps(SSA_steps.back());
 }