[crankshaft] Support ES6 tail call elimination.

src/ast/ast-numbering.cc

Index: src/ast/ast-numbering.cc
diff --git a/src/ast/ast-numbering.cc b/src/ast/ast-numbering.cc
index 272f9bde1128b64e4e7fd0615cd65a837f49f3c6..a09dce02e92fd8317887601a7174d66fbd2ca0bd 100644
--- a/src/ast/ast-numbering.cc
+++ b/src/ast/ast-numbering.cc
@@ -504,9 +504,6 @@ void AstNumberingVisitor::VisitArrayLiteral(ArrayLiteral* node) {
 
 void AstNumberingVisitor::VisitCall(Call* node) {
   IncrementNodeCount();
-  if (node->tail_call_mode() == TailCallMode::kAllow) {
-    DisableOptimization(kTailCall);
-  }
   ReserveFeedbackSlots(node);
   node->set_base_id(ReserveIdRange(Call::num_ids()));
   Visit(node->expression());