[AArch64][PAC] Refactor aarch64-ptrauth pass (#70446)

Refactor Pointer Authentication pass in preparation for adding more
PAUTH_* pseudo instructions:
* dropped early return from runOnMachineFunction() as other PAUTH_*
  instructions need expansion even when pac-ret is disabled
* refactored runOnMachineFunction() to first collect all the
  instructions of interest without modifying anything and then performing
  changes in the later loops. There are two types of relevant
  instructions: PAUTH_* pseudos that should definitely be replaced by this
  pass and tail call instructions that may require attention if pac-ret is
  enabled
* made the loop iterating over all of the instructions handle
  instruction bundles by itself: even though this pass still does not
  support bundled TCRETURN* instructions (such as produced by KCFI) it
  does not crash anymore when no support is actually required