[Attributor][NFC] Expose new API in AAPointerInfo

New users might want to check bins without a load or store instruction
at hand. Since we use those instructions only to find the offset and
size of the access anyway, we can expose an offset and size interface
to the outside world as well.

This commit mainly moves code around and exposes a class (OffsetAndSize)
as well as a method forallInterferingAccesses in AAPointerInfo.

Differential Revision: https://reviews.llvm.org/D119249