[AliasSetTracker] Cleanup addPointer interface. [NFCI]

Summary:
Attempting to simplify the addPointer interface.
Currently there's code decomposing a MemoryLocation into (Ptr, Size, AAMDNodes) only to recreate the MemoryLocation inside the call.

Reviewers: reames, mkazantsev

Subscribers: sanjoy, jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D53836

llvm-svn: 345548