[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 74904a3601569966197a9dd2a8477595b6b853c2..c25a30230ecc19e2f6aa828bb6a4f86cfe8dd7c9 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1416,6 +1416,10 @@ Type* Typer::Visitor::TypeJSInstanceOf(Node* node) {
 
 
 Type* Typer::Visitor::TypeJSLoadContext(Node* node) {
+  ContextAccess const& access = ContextAccessOf(node->op());
+  if (access.index() == Context::EXTENSION_INDEX) {
+    return Type::TaggedPointer();
+  }
   // Since contexts are mutable, we just return the top.
   return Type::Any();
 }