[turbofan] Fix impossible type handling for TypeGuard and BooleanNot.

test/mjsunit/compiler/regress-715651.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function f() {
+  this.x = 1;
+}
+
+var a = [];
+
+// Create enough objects to trigger slack tracking.
+for (let i = 0; i < 100; i++) {
+  new f();
+}
+
+function h() {
+  // Create a new object and add an out-of-object property 'y'.
+  var o = new f();
+  o.y = 1.5;
+  return o;
+}
+
+function g(o) {
+  // Add more properties so that we trigger extension of out-ot-object
+  // property store.
+  o.u = 1.1;
+  o.v = 1.2;
+  o.z = 1.3;
+  // Return a field from the out-of-object-property store.
+  return o.y;
+}
+
+g(h());
+g(h());
+%OptimizeFunctionOnNextCall(g);
+assertEquals(1.5, g(h()));