[LAA] Rework overflow checking in getPtrStride [nfc]

The previous code structure and comments were exceedingly confusing.  I have multiple times looked at this code and suspected a bug.  This time, I decided to take the time to reflow the code and comment out why it is correct.

The only suspect (to me) case left is that an underaligned access with a unit stride (in terms of the access type) might miss the undefined null pointer when wrapping.  This is unlikely to be an issue for C/C++ code with real page sizes, so I'm not bothering to fully convince myself whether that case is correct or not.