Add comment about returning ISD::DELETED_NODE by default

Author: steplong

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -2123,7 +2123,8 @@ class TargetLoweringBase {
   /// Get the ISD node that corresponds to the Instruction class opcode.
   int InstructionOpcodeToISD(unsigned Opcode) const;
 
-  /// Get the ISD node that corresponds to the Intrinsic ID.
+  /// Get the ISD node that corresponds to the Intrinsic ID. Returns
+  /// ISD::DELETED_NODE by default for an unsupported Intrinsic ID.
   int IntrinsicIDToISD(Intrinsic::ID ID) const;
 
   /// @}