fix issue open64-compiler#38 for if-convert

1. the be will crash if compile the code like that:
```
int sre_random(void)
{
  long  rnd1;
  static long  tbl[64];
  int i;
  for (i = 0; i < 64; i++) {
        tbl[i] = rnd1;
        if (tbl[i] < 0)
                tbl[i] += 0;
  }
  return 0;
}
```

Signed-off-by: Zhijin Zeng <[email protected]>

osprey/be/opt/opt_cfg.cxx

@@ -1983,6 +1983,13 @@ CFG::if_convert(WN *wn)
     WN_kid0(store) = sel;
     WN_Set_Linenum(store, WN_Get_Linenum(wn));
 
+    if (empty_then ? then_wn : else_wn) {
+      if (WN * other_stmt = WN_first(empty_then ? then_wn : else_wn)) {
+        INT32 vertex = WN_get_dep_graph_vertex(other_stmt);
+        WN_detach_wn_from_dep_graph(vertex);
+      }
+    }
+
     WN *block = WN_CreateBlock();
     WN_INSERT_BlockFirst(block, store);
     if (if_select_return) {