[turbofan] Support for typed lowering of "prototype" load from functions.

src/compiler/js-typed-lowering.cc

Index: src/compiler/js-typed-lowering.cc
diff --git a/src/compiler/js-typed-lowering.cc b/src/compiler/js-typed-lowering.cc
index 883219d47037d800acff6a80089933324fc9b49a..5482c8d3c15255416766ca8ff42dfb8f72bf60bd 100644
--- a/src/compiler/js-typed-lowering.cc
+++ b/src/compiler/js-typed-lowering.cc
@@ -975,6 +975,27 @@ Reduction JSTypedLowering::ReduceJSLoadNamed(Node* node) {
     ReplaceWithValue(node, value, effect);
     return Replace(value);
   }
+  // Optimize "prototype" property of functions.
+  if (name.is_identical_to(factory()->prototype_string()) &&
+      receiver_type->IsConstant() &&
+      receiver_type->AsConstant()->Value()->IsJSFunction()) {
+    // TODO(turbofan): This lowering might not kick in if we ever lower
+    // the C++ accessor for "prototype" in an earlier optimization pass.
+    Handle<JSFunction> function =
+        Handle<JSFunction>::cast(receiver_type->AsConstant()->Value());
+    if (function->has_initial_map()) {
+      // We need to add a code dependency on the initial map of the {function}
+      // in order to be notified about changes to the "prototype" of {function},
+      // so it doesn't make sense to continue unless deoptimization is enabled.
+      if (!(flags() & kDeoptimizationEnabled)) return NoChange();
+      Handle<Map> initial_map(function->initial_map(), isolate());
+      dependencies()->AssumeInitialMapCantChange(initial_map);
+      Node* value =
+          jsgraph()->Constant(handle(initial_map->prototype(), isolate()));
+      ReplaceWithValue(node, value);
+      return Replace(value);
+    }
+  }
   return NoChange();
 }