[turbofan] Remove some clever-but-wrong bits from select lowering.

test/mjsunit/compiler/regress-600593.js

+// Copyright 2016 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
+
+"use strict"
+
+function f(c) {
+  if (c) { throw new Error(); }
+  throw new Error();
+};
+
+function Error()  {
+  return arguments.length;
+}
+
+assertThrows(function() { f(true); });
+assertThrows(function() { f(false); });
+%OptimizeFunctionOnNextCall(f);
+assertThrows(function() { f(true); });