// Mechanism to get the descriptor for a CALL MachineInstr.
// We get the LLVM CallInstr from the ret. addr. register argument
// of the CALL MachineInstr (which is explicit operand #3 for indirect
// Mechanism to get the descriptor for a CALL MachineInstr.
// We get the LLVM CallInstr from the ret. addr. register argument
// of the CALL MachineInstr (which is explicit operand #3 for indirect