[turbofan] Handle impossible types (Type::None()) in the backend.

test/mjsunit/compiler/regress-630611.js

Index: test/mjsunit/compiler/regress-630611.js
diff --git a/test/mjsunit/regress/regress-crbug-603463.js b/test/mjsunit/compiler/regress-630611.js
similarity index 60%
copy from test/mjsunit/regress/regress-crbug-603463.js
copy to test/mjsunit/compiler/regress-630611.js
index 20bfae65c54fa744d2470903fc92749f99726d67..be75777ba72aeeb0922ac95a0c39f50759e60b85 100644
--- a/test/mjsunit/regress/regress-crbug-603463.js
+++ b/test/mjsunit/compiler/regress-630611.js
@@ -2,15 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-function load(a, i) {
-  return a[i];
-}
+var global = 1;
+global = 2;
 
 function f() {
-  return load(new Proxy({}, {}), undefined);
+  var o = { a : 1 };
+  global = "a";
+  for (var i = global; i < 2; i++) {
+    delete o[i];
+  }
 }
 
 f();
-f();
-load([11, 22, 33], 0);
-f();