[turbofan] Optimize silent hole checks on legacy const context slots.

src/compiler/common-operator-reducer.h

Index: src/compiler/common-operator-reducer.h
diff --git a/src/compiler/common-operator-reducer.h b/src/compiler/common-operator-reducer.h
index 2a814a0bc767eea4290dd27a5f0c9cccf62d13dc..06ead4476bbc4c05f21e67dd40e1a97a21a29be9 100644
--- a/src/compiler/common-operator-reducer.h
+++ b/src/compiler/common-operator-reducer.h
@@ -9,6 +9,11 @@
 
 namespace v8 {
 namespace internal {
+
+// Forward declarations.
+class Factory;
+
+
 namespace compiler {
 
 // Forward declarations.
@@ -37,6 +42,7 @@ class CommonOperatorReducer FINAL : public Reducer {
 
   CommonOperatorBuilder* common() const;
   Graph* graph() const;
+  Factory* factory() const;
   JSGraph* jsgraph() const { return jsgraph_; }
   MachineOperatorBuilder* machine() const;