[turbofan] Introduce CallDescriptorOf helper for safety.

src/compiler/common-operator.cc

Index: src/compiler/common-operator.cc
diff --git a/src/compiler/common-operator.cc b/src/compiler/common-operator.cc
index 72a318c8a543b2e231964fff5909d9890f91e41b..573e4661638a8f3811beefbe28a63c8a47c1d81f 100644
--- a/src/compiler/common-operator.cc
+++ b/src/compiler/common-operator.cc
@@ -98,6 +98,11 @@ SelectParameters const& SelectParametersOf(const Operator* const op) {
   return OpParameter<SelectParameters>(op);
 }
 
+CallDescriptor const* CallDescriptorOf(const Operator* const op) {
+  DCHECK(op->opcode() == IrOpcode::kCall ||
+         op->opcode() == IrOpcode::kTailCall);
+  return OpParameter<CallDescriptor const*>(op);
+}
 
 size_t ProjectionIndexOf(const Operator* const op) {
   DCHECK_EQ(IrOpcode::kProjection, op->opcode());