Simplification of nested pointer arithmetic: improve readability

Use temporaries to ensure const-access where possible and name
subexpressions. This helps navigate the various uses of opX() in this
code.

No change in behaviour is expected. This code is covered by at least 33
of the regression/cbmc/ tests as well as several other regression tests.

Author: tautschnig

src/util/simplify_expr_int.cpp

@@ -439,16 +439,20 @@ simplify_exprt::resultt<> simplify_exprt::simplify_plus(const plus_exprt &expr)
       expr.op0().id() == ID_plus && expr.op0().type().id() == ID_pointer &&
       expr.op0().operands().size() == 2)
     {
-      plus_exprt op0 = to_plus_expr(expr.op0());
+      plus_exprt result = to_plus_expr(expr.op0());
+      const exprt &op1 = as_const(result).op1();
 
-      if(op0.op1().id() == ID_plus)
-        to_plus_expr(op0.op1()).add_to_operands(expr.op1());
+      if(op1.id() == ID_plus)
+      {
+        plus_exprt new_op1 = to_plus_expr(op1);
+        new_op1.add_to_operands(expr.op1());
+        result.op1() = simplify_plus(new_op1);
+      }
       else
-        op0.op1()=plus_exprt(op0.op1(), expr.op1());
-
-      auto result = op0;
-
-      result.op1() = simplify_plus(to_plus_expr(result.op1()));
+      {
+        plus_exprt new_op1{op1, expr.op1()};
+        result.op1() = simplify_plus(new_op1);
+      }
 
       return changed(simplify_plus(result));
     }