[BOLT] Overhaul the comments in PAuthGadgetScanner for readability (NFC) (#169801)
Update the comments in PAuthGadgetScanner.cpp to better describe the
current version of the code. Along the way, shorten identifier names
that are redundant taking their context into account:
`RegsToTrackInstsFor` (made `RegsToTrack`) and `getNumTrackedRegisters`
(made `getNumRegisters`).
Co-authored-by: Kristof Beyls <kristof.beyls@arm.com>