[turbofan] Introduce CallDescriptorOf helper for safety.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 50d74e5943bca71c175a342bf719829c8bc0e66f..a5cf0be302bc6367b858622d49b037416b0c4c3f 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -633,7 +633,7 @@ class RepresentationSelector {
   }
 
   void VisitCall(Node* node, SimplifiedLowering* lowering) {
-    const CallDescriptor* desc = OpParameter<const CallDescriptor*>(node->op());
+    const CallDescriptor* desc = CallDescriptorOf(node->op());
     const MachineSignature* sig = desc->GetMachineSignature();
     int params = static_cast<int>(sig->parameter_count());
     // Propagate representation information from call descriptor.