Make simplify_expr take copy instead of reference

The implementation will make a copy of the argument anyway.
This makes it explicit in the type and allow the use of move by the
caller which avoids a copy.

Author: romainbrenguier

src/util/simplify_expr.cpp

@@ -2574,9 +2574,8 @@ bool simplify(exprt &expr, const namespacet &ns)
   return simplify_exprt(ns).simplify(expr);
 }
 
-exprt simplify_expr(const exprt &src, const namespacet &ns)
+exprt simplify_expr(exprt src, const namespacet &ns)
 {
-  exprt tmp=src;
-  simplify_exprt(ns).simplify(tmp);
-  return tmp;
+  simplify_exprt(ns).simplify(src);
+  return src;
 }

src/util/simplify_expr.h

@@ -25,6 +25,6 @@ bool simplify(
   const namespacet &ns);
 
 // this is the preferred interface
-exprt simplify_expr(const exprt &src, const namespacet &ns);
+exprt simplify_expr(exprt src, const namespacet &ns);
 
 #endif // CPROVER_UTIL_SIMPLIFY_EXPR_H