Revert I+TF for lexical variables

This reverts the following commits to fix a Node.js regression:
* 5529430d "[compiler] Consistently use Ignition+TurboFan for lexical variables."
* 78691367 "[compiler] Improve let+const decision in AstNumbering."

R=adamk@chromium.org, bmeurer@chromium.org, hablich@chromium.org
BUG=
NOTRY=true

Review-Url: https://codereview.chromium.org/2647523002
Cr-Commit-Position: refs/heads/master@{#42484}

src/ast/ast-numbering.cc

@@ -166,12 +166,6 @@ void AstNumberingVisitor::VisitVariableProxyReference(VariableProxy* node) {
     default:
       break;
   }
-  if (node->var()->binding_needs_init()) {
-    // Disable FCG+CS for all variable bindings that need explicit
-    // initialization, i.e. ES2015 style const and let, but not
-    // named function expressions.
-    DisableFullCodegenAndCrankshaft(kReferenceToLetOrConstVariable);
-  }
   node->set_base_id(ReserveIdRange(VariableProxy::num_ids()));
 }
 

src/bailout-reason.h

@@ -174,7 +174,6 @@ namespace internal {
   V(kReceivedInvalidReturnAddress, "Received invalid return address")          \
   V(kReferenceToAVariableWhichRequiresDynamicLookup,                           \
     "Reference to a variable which requires dynamic lookup")                   \
-  V(kReferenceToLetOrConstVariable, "Reference to let or const variable")      \
   V(kReferenceToGlobalLexicalVariable, "Reference to global lexical variable") \
   V(kReferenceToModuleVariable, "Reference to module-allocated variable")      \
   V(kReferenceToUninitializedVariable, "Reference to uninitialized variable")  \

test/mjsunit/regress/regress-v8-5666.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
-
-function foo(j) {
-  for (var i = 0; i < j + 1; i++) {
-    if (i === j) %OptimizeOsr();
-  }
-  let k = 1
-}
-
-foo(1000);
-foo(1000);
-%OptimizeFunctionOnNextCall(foo);
-foo(-1);
-assertOptimized(foo);