[turbofan] Do not rename context for loop exits in bytecode graph builder.

Renaming context confuses specialization passes. (And looking through
loop exit renames does not help because we insert Phis for SSA.)

BUG=chromium:635808

Review-Url: https://codereview.chromium.org/2221363002
Cr-Commit-Position: refs/heads/master@{#38517}

src/compiler/bytecode-graph-builder.cc

@@ -413,8 +413,9 @@ void BytecodeGraphBuilder::Environment::PrepareForLoopExit(Node* loop) {
                                          GetEffectDependency(), loop_exit);
   UpdateEffectDependency(effect_rename);
 
-  // Rename the current context.
-  context_ = graph()->NewNode(common()->LoopExitValue(), context_, loop_exit);
+  // TODO(jarin) We should also rename context here. However, uncoditional
+  // renaming confuses global object and native context specialization.
+  // We should only rename if the context is assigned in the loop.
 
   // Rename the environmnent values.
   for (size_t i = 0; i < values_.size(); i++) {