[turbofan] Introduce new Select operator to improve bounds checking.

src/compiler/js-generic-lowering.cc

Index: src/compiler/js-generic-lowering.cc
diff --git a/src/compiler/js-generic-lowering.cc b/src/compiler/js-generic-lowering.cc
index c16aa82bafaad8e3af459df12a8099a7d08a8b61..11979ec6edc9ea800980354a4ca70521fe7bddaa 100644
--- a/src/compiler/js-generic-lowering.cc
+++ b/src/compiler/js-generic-lowering.cc
@@ -64,6 +64,7 @@ Reduction JSGenericLowering::Reduce(Node* node) {
     Lower##x(node);     \
     break;
     DECLARE_CASE(Branch)
+    DECLARE_CASE(Select)
     JS_OP_LIST(DECLARE_CASE)
 #undef DECLARE_CASE
     default:
@@ -241,6 +242,23 @@ void JSGenericLowering::LowerBranch(Node* node) {
 }
 
 
+void JSGenericLowering::LowerSelect(Node* node) {
+  // TODO(bmeurer): This should probably be moved into a separate file.
+  SelectParameters const& p = SelectParametersOf(node->op());
+  Node* branch = graph()->NewNode(common()->Branch(p.hint()), node->InputAt(0),
+                                  graph()->start());
+  Node* if_true = graph()->NewNode(common()->IfTrue(), branch);
+  Node* vtrue = node->InputAt(1);
+  Node* if_false = graph()->NewNode(common()->IfFalse(), branch);
+  Node* vfalse = node->InputAt(2);
+  Node* merge = graph()->NewNode(common()->Merge(2), if_true, if_false);
+  node->set_op(common()->Phi(p.type(), 2));
+  node->ReplaceInput(0, vtrue);
+  node->ReplaceInput(1, vfalse);
+  node->ReplaceInput(2, merge);
+}
+
+
 void JSGenericLowering::LowerJSUnaryNot(Node* node) {
   Callable callable = CodeFactory::ToBoolean(
       isolate(), ToBooleanStub::RESULT_AS_INVERSE_ODDBALL);