[Attributor] Introduce getPotentialCopiesOfStoredValue and use it

This patch introduces `getPotentialCopiesOfStoredValue` which uses
AAPointerInfo to determine all "aliases" or "potential copies" of a
value that is stored into memory. This operation can fail but if it
succeeds it means we can visit all "uses" of a value even if it is
temporarily stored in memory.

There are two users for the function:
  1) `Attributor::checkForAllUses` which will now ignore the value use
     in a store if all "potential copies" can be identified and instead
     be visited. This allows various AAs, including AAPointerInfo
     itself, to look through memory.
  2) `AANoCapture` which uses a custom use tracking through the
     CaptureTracker interface and therefore needs to be thought
     explicitly.

Differential Revision: https://reviews.llvm.org/D106185

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