[turbofan] Simplify reduction if IfTrue and IfFalse and fix bugs.

test/mjsunit/regress/regress-451958.js

+// Copyright 2015 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 k() { throw "e"; }
+var a = true;
+var a = false;
+function foo(a) {
+  var i, j;
+  if (a) {
+    for (i = 0; i < 1; j++) ;
+    for (i = 0; i < 1; k()) ;
+    for (i = 0; i < 1; i++) ;
+  }
+}
+%OptimizeFunctionOnNextCall(foo);
+foo();
+
+function bar() {
+var __v_45;
+  for (__v_45 = 0; __v_45 < 64; __v_63++) {
+  }
+  for (__v_45 = 0; __v_45 < 128; __v_36++) {
+  }
+  for (__v_45 = 128; __v_45 < 256; __v_45++) {
+  }
+}
+%OptimizeFunctionOnNextCall(bar);
+assertThrows(bar);