[CodeGen] Add unittest for findDebugLoc, rfindDebugLoc, findPrevDebugLoc and rfindPrevDebugLoc. NFC

- Add some unittests for the findDebugLoc, rfindDebugLoc,
  findPrevDebugLoc and rfindPrevDebugLoc helpers in MachineBasicBlock.

- Clean up code comments and code formatting related to the functions
  mentioned above.

This was extracted as a pre-commit to D150577, adn some of the tests
are commented out since they would crash/assert in a rather
uncontrolled way.