Fix ICE with -Wduplicated-branches (PR c/79199)

[Marek Polacek]

This testcase was breaking because we found ourselves in operand_equal_p
with a COND_EXPR whose operand 2 was null -- i.e., missing else branch,
which won't happen for ? : but can happen here via -Wduplicated-branches.

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

2017-01-26  Marek Polacek  <pola...@redhat.com>

        PR c/79199
        * fold-const.c (operand_equal_p) [COND_EXPR]: Use OP_SAME_WITH_NULL
        for the third operand.

        * c-c++-common/Wduplicated-branches-13.c: New test.

diff --git gcc/fold-const.c gcc/fold-const.c
index 5576d59..a8bb8af 100644
--- gcc/fold-const.c
+++ gcc/fold-const.c
@@ -3147,7 +3147,7 @@ operand_equal_p (const_tree arg0, const_tree arg1, 
unsigned int flags)
                                      TREE_OPERAND (arg1, 0), flags));
 
        case COND_EXPR:
-         if (! OP_SAME (1) || ! OP_SAME (2))
+         if (! OP_SAME (1) || ! OP_SAME_WITH_NULL (2))
            return 0;
          flags &= ~OEP_ADDRESS_OF;
          return OP_SAME (0);
diff --git gcc/testsuite/c-c++-common/Wduplicated-branches-13.c 
gcc/testsuite/c-c++-common/Wduplicated-branches-13.c
index e69de29..7aa5b37 100644
--- gcc/testsuite/c-c++-common/Wduplicated-branches-13.c
+++ gcc/testsuite/c-c++-common/Wduplicated-branches-13.c
@@ -0,0 +1,23 @@
+/* PR c/79199 */
+/* { dg-do compile } */
+/* { dg-options "-Wduplicated-branches" } */
+
+unsigned int a, b, c, d, e;
+void
+fn1 (void)
+{
+  if (0) /* { dg-warning "this condition has identical branches" } */
+    {
+      if (d > 4294967293)
+       (void) 5;
+      c = d;
+      b = e | a;
+    }
+  else
+    {
+      if (d > 4294967293)
+       (void) 5;
+      c = d;
+      b = e | a;
+    }
+}

        Marek