Small C++ tweak to fold_simple

[Marek Polacek]

fold_simple struck me as odd, certainly the NULL_TREE assignment is
unnecessary, so this patch simplifies it a bit.

I've been confused about the commentary too, what is the difference between
"constant-expressions" and "constexpressions"?  We should clarify that.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-01-19  Marek Polacek  <pola...@redhat.com>

        * constexpr.c (fold_simple): Simplify.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 9a548d29bbc..ca7f369f7e9 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -4931,22 +4931,21 @@ fold_simple_1 (tree t)
 }
 
 /* If T is a simple constant expression, returns its simplified value.
-   Otherwise returns T.  In contrast to maybe_constant_value do we
+   Otherwise returns T.  In contrast to maybe_constant_value we
    simplify only few operations on constant-expressions, and we don't
    try to simplify constexpressions.  */
 
 tree
 fold_simple (tree t)
 {
-  tree r = NULL_TREE;
   if (processing_template_decl)
     return t;
 
-  r = fold_simple_1 (t);
-  if (!r)
-    r = t;
+  tree r = fold_simple_1 (t);
+  if (r)
+    return r;
 
-  return r;
+  return t;
 }
 
 /* If T is a constant expression, returns its reduced value.

        Marek