Use to_plus_expr instead of assert in simplify_plus

to_plus_expr should check if an expression is a plus_exprt, this avoids
duplicating checks across the codebase

Author: hannes-steffenhagen-diffblue

src/util/simplify_expr_int.cpp

@@ -446,6 +446,8 @@ bool simplify_exprt::simplify_mod(exprt &expr)
 
 bool simplify_exprt::simplify_plus(exprt &expr)
 {
+  // only used to check structural invariants
+  (void)to_plus_expr(expr);
   if(!is_number(expr.type()) &&
      expr.type().id()!=ID_pointer)
     return true;
@@ -454,8 +456,6 @@ bool simplify_exprt::simplify_plus(exprt &expr)
 
   exprt::operandst &operands=expr.operands();
 
-  assert(expr.id()==ID_plus);
-
   // floating-point addition is _NOT_ associative; thus,
   // there is special case for float