c++: ICE when shortening right shift [PR94955]

Since r10-6527 fold_for_warn calls maybe_constant_value, which means it
can fold more than it previously could.  In this testcase it means that
cp_build_binary_op/RSHIFT_EXPR set short_shift because now we were able
to fold op1 to an INTEGER_CST.  But then when actually performing the
shortening we crashed because cp_fold_rvalue wasn't able to fold as much
as f_f_w and so tree_int_cst_sgn crashed on a NOP_EXPR.  Therefore the
calls should probably match.

	PR c++/94955
	* typeck.c (cp_build_binary_op): Use fold_for_warn instead of
	cp_fold_rvalue.

	* g++.dg/cpp0x/constexpr-shift2.C: New test.

gcc/cp/typeck.c

@@ -5622,7 +5622,9 @@ cp_build_binary_op (const op_location_t &location,
 	{
 	  int unsigned_arg;
 	  tree arg0 = get_narrower (op0, &unsigned_arg);
-	  tree const_op1 = cp_fold_rvalue (op1);
+	  /* We're not really warning here but when we set short_shift we
+	     used fold_for_warn to fold the operand.  */
+	  tree const_op1 = fold_for_warn (op1);
 
 	  final_type = result_type;
 

gcc/testsuite/g++.dg/cpp0x/constexpr-shift2.C

+// PR c++/94955
+// { dg-do compile { target c++11 } }
+
+struct S {
+  static constexpr char foo() { return 10; }
+};
+
+short int
+fn (short int e)
+{
+  return e >> S::foo();
+}