[M68k][MIR](2/8) Changes in the target-independent MIR part

 - Add new callback in `TargetInstrInfo` --
  `isPCRelRegisterOperandLegal` -- to query whether pc-rel
   register MachineOperand is legal.
 - Add new function to search DebugLoc in a reverse ordering

Authors: myhsu, m4yers, glaubitz

Differential Revision: https://reviews.llvm.org/D88386